This forum thread discusses the issue and includes various solutions/workarounds, such as deleting
Rather than removing the akonadi configuration, I edited
~/.config/akonadi/aknoadiserverrc and changed
StartServer=false, and then rebooted (although logging out and back in should have been sufficient). (1)
or this one
I had the same problem here, also on a Kubuntu system that has undergone many distribution upgrades.
I found the following solution without having to delete Akonadi's configuration or disabling it completely:
In the "Akonadi configuration" dialog (where you configure the Akonadi ressources), I had an e-mail ressource named "Local Folder". Deleting it made the startup warning go away. (2)