I had to create the directory /var/db/mysql, make that writable for anyone who uses KDE (in my case only me, so I just chown'ed it).
Then deleted ~/.local/share/akonadi and ~/.config/akonadi, restarted KDE and no more errors. Of course I did have to re-enter my calendar resources from KDE 4.4 They were no longer shown, even though their contents were still intact.
Also note I was out of KDE or any other desktop environment when deleting the akonadi-directories.