before I explain my problem, I am most humbly asking for one small favour:
Please(!!) nobody give me any advice along the line of: Just do a factory reset and start from scratch
I've got almost a thousand apps installed on my system and I'm in no mood to even try to reconfigure it all over again.
Some time ago I installed the latest beta of Cyanogenmod on my Samsung Galaxy Note Tablet SM-P900.
After some time I encountered so many quirks with it, that I decided to flash back to the latest stable build 12.1-20151029 (Android v5.1.1).
Since I was always able to upgrade w/o wiping data, I did the same here, but right after the downgrade the system started misbehaving.
Matter of fact, I could only boot through to the desktop, once I had removed some google.* related dirs from the data/data location. Right afterwards I reinstalled the affected apps from backup and that fixed that.
Unfortunately during the proces of peeking and poking in data/data I messed up some ownership/permission settings.
I also managed to fix those via an old backup (via: chmod/chown --reference), *but* now I am getting a popup message telling me:
"There is something wrong with your data partition..." everytime I reboot the system.
Afterwards I can work just fine with the tablet, but every 3rd or 4th day it suddenly reboots - then being stable again for another couple of days.
If I could only find out what fault this messages refers to, I could fix it manually - but I couldn't find anything useful by reviewing the logcat info.