I've set Kontact to autostart after login, and after every boot I get the message:

No resource agents have been found, Akonadi is not usable without at least one. This usually means that no resource agents are installed or that there is a setup problem. The following paths have been searched: '/usr/share/akonadi/agents'. The XDG_DATA_DIRS environment variable is set to '/usr/share:/usr/local/share:/opt/kde/share', make sure this includes all paths where Akonadi agents are installed to.

If I restart Kontact or if I log out and then back in I get no messages. Also, restarting Akonadi or running the Akonadi server test manually goes as should, so no error messages there.

I've tested different XDG_DATA_DIRS values, and the directory /usr/share/akonadi/agents exists and there are several .desktop files.

I've also deleted all kontact files and folders under .kde and .kde4.

There are no other error messages, only the one about resource agents during the first start of Kontact after boot.

I'm using KDE 4.4.

Please read http://userbase.kde.org/Akonadi - specifically the parts about addressbooks. It sounds as though you have not yet migrated your addressbook. It might be worth trying opening 'kaddressbook' as a stand-alone. If that succeeds, it will automatically convert your addressbook to an Akonadi-controlled one. Let us know whether you still see the error after that.

Thanks for your answer!

It seems nothing else helped, but in System Settings -> Advanced -> KDE Resources I added an Akonadi based contact resource and made it the standard resource. This worked and I no longer see the error message.

Funny thing is I don't even use contact resources and I have disabled Contacts plugin in Kontact.

Annew's suggestion worked a treat for me,
thanks,
Ian

That seems pretty good to me and I hope you will keep up the good work in future as well.