Kontact doesn't show anything anymore

Kontact and/or akonadi has a problem. It doesn’t show anything anymore. If I click on any folder within the email component, it starts showing a gearwheel that just keeps on spinning, contacts component show those gearwheels all over my contact subfolders, and the calendar/tasks component doesn’t show anything at all.

Starting akonadiconsole shows that the relevant agents are busy doing something, but there’s no progress at all, and manually triggering a sync doesn’t do anything, nor does restarting.
Restarting all of akonadi doesn’t help either (akonadictl stop/start). After a reboot it will work for a while, but then the problems reemerge.

So obviously, there is a problem, but I would like to narrow it down before reporting the bug, as it seems to be all over the place. How would I do that?