Not sure if this will help but you could try this
Wiith KMail/Kontact closed, make sure the Akonadi server is running (you can check it by running akonaditray or by going to Config Your Desktop > Workspace Appearance and Behavior > Akonadi Configuration) and check on the Akonadi Server Configuration tab
After that switch to the other tab Akonadi Resources Configuration
and select Add to add a mail directory.
After that's done try restarting KMail/Kontact