Thank you very much for your reply Olav,
I really don't think that this is an issue of a real change in temperature. With the former used Marlin Firmware I've seen stable (room-) temperatures on all thermistors. And all 3 thermistors showed the same temp.
Now the value changes twice a second from 24 to 25 and back. (in fact... the lcd temperature changes much faster than the temperature shown in Repetier Host when the Printer is running)
The developers Edition Firmware is really tricky - in my opinion. There are so many things different to the normal release. I simply don't know where to search first.
Probably it would be a good idea to just integrate the mesh-bed-leveling into my "old" Firmware Release. But I can not just integrate mesh_bed_leveling.h and mesh_med_leveling.cpp into my sketch. It won't compile.
I will try to do a long print for several hours and hope that the Printer don't fail.
Hope it works either way.
Thanks again Olav