Thank you for your help.
I reflashed the ROM few days ago, reinstall most of my apps, restore some others with Titanium Backup and use Link2SD to move all of them to the SD card.
Today, I have the same message: "Insufficient storage available" when I try to update an app from the Play Store.
Here are my Link2SD Storage Info :
Internal: 64% Free - Used: 357MB - Free: 651MB
SD Card: 37% Free - Used: 8.26GB - Free: 5.05GB
SD Card 2nd Part. : not found
System: 54% Free - Used: 231MB - Free: 273MB
Cache: 99% Free - Used: 3.03MB - Free: 466MB
In SlimBean ROM, for instance, Gmail is a system app, I tried to clean the cache of this app, but still the same error message...