Thanks for all the reports and beacons I received over the last 3 weeks.
The Voltage has never climbed over 3 volts (in full sunlight). We can assume that COMPASS is totally OFF during eclipses.
If COMPASS enters the sunlight she will reboot. Because the low voltage COMPASS is switching into emergency mode.
That means, that CDHS (Command & Data Handling Subsystem) and ADCS (Attitude Determination and Control Subsystem) units are completely off. Unfortunately this means also, that we CAN NOT change the threshold values by commands.
That's the main problem. It seems the satellite is "locked" because in emergency mode she will accept only two commands: hard reset and TX enable. Both commands are impractical for recovery.
The only thing we can do is WAIT. Wait for longer sun periods and voltages over 3.47 Volts. That's the standard threshold value in emergency mode to switch into powersafe mode. Only in this mode we can change the threshold values.
We may have lost a cubesat, but we've definitely got some new friends :-) Thank you all for your efforts!