Hi Vera Ustinov

Because you say "the segment won´t be saved as translated" I will assume that that the target segments are not changing from draft to translated. 
My question is - are the translations being saved into the TM?

I also assume this is the only project/file that is causing such strange behaviour?
If yes, can you do the following:

1. Locate the original source file
2. Create new project
3. Pre-Translate using your TM

From my limited perspective, if this is isolated to one project/file the files themselves maybe compromised.
Seeing if recreating the project would be interesting.

Looking forward to your confirmation

Lydia 

Dear Lydia,

Thank you very much again for your reply :-) The suggested solution has resolved the error message but also led to the loss of around 1500 already translated words which didn´t matter because I tested the solution only after having completed the project in question.

Kind regards
Vera

Dear Lydia,

Thank you very much again for your reply :-) The suggested solution has resolved the error message but also led to the loss of around 1500 already translated words which didn´t matter because I tested the solution only after having completed the project in question.

Kind regards
Vera