This was the real problem.
The Game Flight mode was been activated for an unknown reason.
After deactivating this option, everything returned to normal.
Can this option be activated during an update, because I have never used it before?
Thanks to everyone for the good advice.
Now I can fly happily again in my L-39 module. :pilotfly:
PS: I tested with "Game Flight Mode on and off and this was the real cause.
Strange that a "Game Flight Mode" flies more difficult than the real Flight mode. :smilewink:
Grippen7's post in Is something wrong with the L-39 flaps and trim system? was marked as the answer