sorry, I found it:
After the Update to Joomla 3.10.1 (and/ or to HU 2.0.1, I don't know) all Targets from first Language (in my case: German) to the second Language (here: English) seemed to be overweitten in the assigment of the Template Style :
Normally there has to be an assigment to the seperate english Template Style (in my Case Helix Ultimate "EN" - but it seemed to be that all targets in the Template Style Association switched to "Default".
However, after manually correct all items, it works..
So you were right, Ms. Rashida, Thank you again and sorry.
I hope that this will help others, if they have similar problems.