If I try to uninstall Cerberus, I get this: “Deinstallation nicht möglich: Dieses Paket ist ein aktiver Geräteadministrator.” (It is not possible to uninstall this. The package is an active system administrator) Even if I deactivated it beforehand.
But that doesn’t work. In fact it says that the device administrator is already deactivated. Nevertheless it is not possible to uninstall Cerberus. Not via pulling the icon in the trash can, not via google play.
Well, there wouldn’t be much point to Cerberus if it was easy to remove.
That being said, cerberus list two methods of stopping the administrator function in their FAQ. If you’ve succesfully disabled the system administration feature, it should be possible to remove it via Settings > Apps.
Not sure whether that will solve all your problems though - your problems sound similar to those reported by different users here:
It could be worth doing a hard reset on the phone, but make sure you make a backup of any data you want to save. The built-in backup and restore app does NOT backup all data. See the topic above for some discussion of backup options, or check Fairphone’s support pages, or use the search function on the forum to find relevant topics.