Update from my side. I did the upgrade to QTS 5 but the issue came up again. I tried the fix from Roon Server on QNAP QTS 4.5.1 Situation - #21 by crieke which seems to be very user friendly. Didn’t work and the ffmpeg folder in my Roon directory got deleted by the system after rebooting, god knows why.
Then I did the same thing again PLUS followed I can not play AAC format file, why? - #16 by Daniel_Murphy from @Daniel_Murphy , copied the ffmpeg directly through cyberduck into my /usr/local/medialibrary/bin .
Restarted the Roon on my Mac (not Server) aaaaannnddd … thank god it works again. No idea what now finally the fix was, but it works.
I might not upgrade my Qnap for the coming six month lol …