Thx,
i tried the Microsoft Font Validator, but as i'm only a user of fonts and do not have a glue about font internas i'm unable to interpret the result. There were some yellow and red areas, but the red areas are pointing to versioning and signatures only.
Finally
converting from otf to ttf did the trick. Still the validator has some issues, but the signature error is gone.
Interesting that different Qt versions/platforms do have a problem with the font and others not.
Thx everyone for helping.
r:/ubeus