uthier,
Are you running the stock firmware?
You could try booting into Recovery mode (with the phone turned completely off, press and hold the Home button AND Volume Up button AND Power button, and don't release them until you see the splash screen) and clearing the cache.
Or you could remove your Google account and add it back.
Can you post a screenshot of your Storage view?
If all else fails, it's time for a Factory Reset.
Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM -0.001 using Tapatalk 4