Good news.
Sadly, this is a Thief 1-FM, so i can't run it in the moment. (black screen or freeze, tried all the patches)
sNeaksieGarrett gave me an advice to use ddfix. I always thought, it is for Thief 2 only. But it is as simple as copying the ddfix-files to the thief1-directory and hex-editing the thief.exe. And it WORKED !!!
Many thanks to sNeaksieGarrett for telling me and timeslip for the ddfix.