This is on KDE 4.12 on ARch (fully upgraded):

After I had problems with akonadi mysql socket (again) I could fix it by removing the .local/share/akonadi folder.

Now everything works again apart from kmail ...
When I start kmail I get the following message box with a serious error:
"unable to execute collection query" ...
when I close this message box kmail gets closed also ...

I don't find any documentation on this error and I have no idea how to get the correct info out of the akonadi console debugging.
My guess is it has something to do with searches I saved in kmail before (cause they are gone now!) but I don't know how to clear this anyway.

Also since there is 12 email accounts configured with folder archiving and many personalized settings I don't want to set it all up again.

Plz, I'd really appreciate if someone could help me to fix this or at least to isolate the problem ...

thx for reading,
piedro

Try this:
Run akonadiconsole, switch to the "Browser" tab, right-click on each collection (or select all of them and right-click on the selection) and choose "Synchronize" or "Synchronize Recursively".

You may have to set your resources to online first on the "Agents" tab.