First time I started kmail2 under 12.04, the automatic conversion of folders failed and a message suggested to run kmail-migrator --interactive. This just said "Migration of kmailrc has already run, not running it again."
If I run kmail now, the launching icon appears, then nothing happens. running it once more opens up the Kmail window and an error window KMail encountered a fatal error and will terminate now. The error was: unknown resource.
Restarting akonadi did not help. Running kmail from a terminal gives

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QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
Known subscriber "kmail2" subscribes again
Known subscriber "kmail2" subscribes again
Database "akonadi" opened using driver "QMYSQL"
Database "akonadi" opened using driver "QMYSQL"

"/usr/bin/kmail(15864)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(15864)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(15864)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(15864)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(15864)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(15864)" Soprano: "Invalid iterator."
"/usr/bin/kmail(15864)" Soprano: "QLocalSocket::connectToServer: Invalid name"
Known subscriber "kmail2" subscribes again
Known subscriber "kmail2" subscribes again
kmail2(15864)/kdepimlibs (mailtransport): Could not access Outbox.
jan@haddock:~$
jan@haddock:~$ "/usr/bin/kmail(15864)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(15864)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(15864)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(15864)" Soprano: "Invalid iterator."
kmail2(15864)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
kmail2(15864)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
kmail2(15864) MailCommon::Kernel::emergencyExit: "KMail encountered a fatal error and will terminate now.
The error was:
Unknown resource"

Nepomuk Semantic Desktop is enabled.

How can I set this up properly?

What do you mean by 12.04? Ubuntu 12.04?

Yes, more precisely, kubuntu 12.04.

Kubuntu 12.04 is an extremely old distribution: what version of kmail2 are you using? (kmail --version should tell)

Qt: 4.8.1
KDE Development Platform: 4.8.5 (4.8.5)
KMail: 4.8.5

I tried reinstalling kmail - no change at all.

Some more info: after a some tries stopping/restarting akonadi, at some point I am getting this error (excerpt):

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Database process exited unexpectedly during initial connection!
executable: "/usr/sbin/mysqld-akonadi"
arguments: ("--defaults-file=/home/jan/.local/share/akonadi/mysql.conf", "--datadir=/home/jan/.local/share/akonadi/db_data/", "--socket=/home/jan/.local/share/akonadi/socket-haddock/mysql.socket")
stdout: ""
stderr: "Could not open required defaults file: /home/jan/.local/share/akonadi/mysql.conf
Fatal error in defaults handling. Program aborted
...
ProcessControl: Application 'akonadiserver' returned with exit code 255 (Unknown error)
"akonadiserver" crashed too often and will not be restarted!


At this point, I try running kmail. The start of the error message is the same as before, but there is much more:

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QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
kmail2(24065)/libakonadi Akonadi::SessionPrivate::socketError: Socket error occurred: "QLocalSocket::connectToServer: Socket access error"
kmail2(24065)/libakonadi Akonadi::SessionPrivate::socketError: Socket error occurred: "QLocalSocket::connectToServer: Socket access error"
Known subscriber "kmail2" subscribes again
Known subscriber "kmail2" subscribes again
"/usr/bin/kmail(24065)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(24065)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(24065)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(24065)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(24065)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(24065)" Soprano: "Invalid iterator."
"/usr/bin/kmail(24065)" Soprano: "QLocalSocket::connectToServer: Invalid name"
Known subscriber "kmail2" subscribes again
Known subscriber "kmail2" subscribes again
kmail2(24065)/kdepimlibs (mailtransport): Could not access Outbox.
jan@haddock:~$ "/usr/bin/kmail(24065)" Soprano: "QLocalSocket::connectToServer: Invalid name"
"/usr/bin/kmail(24065)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(24065)" Soprano: "Unsupported operation (2)": "Invalid model"
"/usr/bin/kmail(24065)" Soprano: "Invalid iterator."
Known subscriber "akonadi_pop3_resource_5" subscribes again
Known subscriber "akonadi_pop3_resource_1" subscribes again
Known subscriber "akonadi_pop3_resource_2" subscribes again
Known subscriber "akonadi_pop3_resource_4" subscribes again
Known subscriber "akonadi_pop3_resource_0" subscribes again
Known subscriber "akonadi_pop3_resource_3" subscribes again
akonadi_pop3_resource_5(21546)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
akonadi_pop3_resource_5(21546)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
akonadi_pop3_resource_5(21546)/akonadiresource (pop3) POP3Resource::cancelSync: ============== ERROR DURING POP3 SYNC ==========================
akonadi_pop3_resource_5(21546)/akonadiresource (pop3) POP3Resource::cancelSync: "Error while trying to get the local inbox folder, aborting mail check.
Unknown error. (Unknown resource)"
akonadi_pop3_resource_5(21546)/akonadiresource (pop3) POP3Resource::saveSeenUIDList: UID list from server is not valid.
akonadi_pop3_resource_5(21546)/libakonadi Akonadi::SpecialCollectionsRequestJobPrivate::resourceScanResult: Failed to request resource "akonadi_maildir_resource_0" : "Unknown error. (Unknown resource)"
akonadi_pop3_resource_5(21546)/libakonadi Akonadi::DefaultResourceJobPrivate::collectionFetchResult: "Unknown resource"
akonadi_pop3_resource_3(21544)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
akonadi_pop3_resource_3(21544)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
akonadi_pop3_resource_4(21545)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
akonadi_pop3_resource_4(21545)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
akonadi_pop3_resource_3(21544)/akonadiresource (pop3) POP3Resource::cancelSync: ============== ERROR DURING POP3 SYNC ==========================
akonadi_pop3_resource_3(21544)/akonadiresource (pop3) POP3Resource::cancelSync: "Error while trying to get the local inbox folder, aborting mail check.
Unknown error. (Unknown resource)"
akonadi_pop3_resource_3(21544)/akonadiresource (pop3) POP3Resource::saveSeenUIDList: UID list from server is not valid.
akonadi_pop3_resource_3(21544)/libakonadi Akonadi::SpecialCollectionsRequestJobPrivate::resourceScanResult: Failed to request resource "akonadi_maildir_resource_0" : "Unknown error. (Unknown resource)"
akonadi_pop3_resource_3(21544)/libakonadi Akonadi::DefaultResourceJobPrivate::collectionFetchResult: "Unknown resource"
akonadi_pop3_resource_4(21545)/akonadiresource (pop3) POP3Resource::cancelSync: ============== ERROR DURING POP3 SYNC ==========================
akonadi_pop3_resource_4(21545)/akonadiresource (pop3) POP3Resource::cancelSync: "Error while trying to get the local inbox folder, aborting mail check.
Unknown error. (Unknown resource)"
akonadi_pop3_resource_4(21545)/akonadiresource (pop3) POP3Resource::saveSeenUIDList: UID list from server is not valid.
akonadi_pop3_resource_4(21545)/libakonadi Akonadi::SpecialCollectionsRequestJobPrivate::resourceScanResult: Failed to request resource "akonadi_maildir_resource_0" : "Unknown error. (Unknown resource)"
akonadi_pop3_resource_4(21545)/libakonadi Akonadi::DefaultResourceJobPrivate::collectionFetchResult: "Unknown resource"
akonadi_pop3_resource_2(21543)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
akonadi_pop3_resource_2(21543)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
akonadi_pop3_resource_2(21543)/akonadiresource (pop3) POP3Resource::cancelSync: ============== ERROR DURING POP3 SYNC ==========================
akonadi_pop3_resource_2(21543)/akonadiresource (pop3) POP3Resource::cancelSync: "Error while trying to get the local inbox folder, aborting mail check.
Unknown error. (Unknown resource)"
akonadi_pop3_resource_2(21543)/akonadiresource (pop3) POP3Resource::saveSeenUIDList: UID list from server is not valid.
akonadi_pop3_resource_2(21543)/libakonadi Akonadi::SpecialCollectionsRequestJobPrivate::resourceScanResult: Failed to request resource "akonadi_maildir_resource_0" : "Unknown error. (Unknown resource)"
akonadi_pop3_resource_2(21543)/libakonadi Akonadi::DefaultResourceJobPrivate::collectionFetchResult: "Unknown resource"
akonadi_pop3_resource_1(21542)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
akonadi_pop3_resource_1(21542)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
akonadi_pop3_resource_0(21541)/libakonadi Akonadi::DefaultResourceJob::slotResult: "Unknown resource"
akonadi_pop3_resource_0(21541)/libakonadi Akonadi::SpecialCollectionsRequestJob::slotResult: Failed SpecialCollectionsRequestJob::slotResult "Unknown error. (Unknown resource)"
akonadi_pop3_resource_0(21541)/akonadiresource (pop3) POP3Resource::cancelSync: ============== ERROR DURING POP3 SYNC ==========================
akonadi_pop3_resource_0(21541)/akonadiresource (pop3) POP3Resource::cancelSync: "Error while trying to get the local inbox folder, aborting mail check.
Unknown error. (Unknown resource)"
akonadi_pop3_resource_0(21541)/akonadiresource (pop3) POP3Resource::saveSeenUIDList: UID list from server is not valid.
akonadi_pop3_resource_0(21541)/libakonadi Akonadi::SpecialCollectionsRequestJobPrivate::resourceScanResult: Failed to request resource "akonadi_maildir_resource_0" : "Unknown error. (Unknown resource)"
akonadi_pop3_resource_1(21542)/akonadiresource (pop3) POP3Resource::cancelSync: ============== ERROR DURING POP3 SYNC ==========================
akonadi_pop3_resource_1(21542)/akonadiresource (pop3) POP3Resource::cancelSync: "Error while trying to get the local inbox folder, aborting mail check.
Unknown error. (Unknown resource)"
akonadi_pop3_resource_1(21542)/akonadiresource (pop3) POP3Resource::saveSeenUIDList: UID list from server is not valid.
akonadi_pop3_resource_1(21542)/libakonadi Akonadi::SpecialCollectionsRequestJobPrivate::resourceScanResult: Failed to request resource "akonadi_maildir_resource_0" : "Unknown error. (Unknown resource)"
akonadi_pop3_resource_0(21541)/libakonadi Akonadi::DefaultResourceJobPrivate::collectionFetchResult: "Unknown resource"
akonadi_pop3_resource_1(21542)/libakonadi Akonadi::DefaultResourceJobPrivate::collectionFetchResult: "Unknown resource"

The difference is that now, kmail is running and there is no error window. However, when checking mail, there is a (multiple repeated) system message (lower left of screen) "error while trying to get the local inbox folder, aborting mail check. (Unknown resource)"

When trying to compose a new message, the message form pops up but shows an error:

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You do not have the semantic desktop system enabled.  The following features will not work correctly:
- Recipient auto-completion
- Distribution lists
- Per-contact crypto preferences

This looks like https://bugs.kde.org/show_bug.cgi?id=294320. Also,

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pgrep -fl nepomuk
1866 /usr/bin/nepomukserver
1873 /usr/bin/nepomukservicestub nepomukstorage
2013 /usr/bin/nepomukcontroller -session 10d7c4cf64000145031456900000016270017_1450583477_471157
21540 /usr/bin/akonadi_nepomuk_feeder --identifier akonadi_nepomuk_feeder

As far as I can tell, this bug is still open.

The version shipped by Ubuntu 12.04 LTS is extremely old and has bugs that have been already fixed in more recent versions. If possibile you should try to upgrade to at least 14.04 LTS version.

Just finished installing 14.04. kmail is working now; conversion of old messages seems OK. Thanks.