The problem founded on a bi-lingual survey. The mailing text was stored only for one language version, while all respondents in the address list were assigned to the other language. The other language, indeed, had the default text stored in the mailing.
Please switch between the language versions in the mailing, and copy the correct text into both langage version.
The problem was somewhat harrd to see, as the languages are "Deutsch (Sie)" and "Deutsch (Du)". And, of course your're welcome to ask question in the online support in German, as well. Wir sitzen immerhin in München.