i found the problem. the ffmpeg binary form the zip file has no execute rights:
-rwxr--r-- 1 user user 6816988 9 Jan 23:29 ffmpeg
i changed the rights to the same values as the original ffmpeg file has:
-rwxr-xr-x 1 user user 6816988 9 Jan 23:29 ffmpeg
now it works :-).
thanks a lot