OK, this issue is solved.
I extracted the comments of the TeX file and added them to the PO (as PO
comments). These comments were no re-encoded.
I now encode them in the TeX module. I think it is better not to encode
the comments in Transtractor because the comment encoding is not known
there (it is not necessary the input charset, as in my case).
Sorry for the noise,
--
Nekral