Hi,
I encouter the same issue.
I downloaded ffmpeg for amd64 architecture from johnvansickles website ffmpeg-git-amd64-static.tar.xz - md5
I extracted the ffmpeg file, copied it to my RoonOnNAS/bin folder (which was emptied) and restarted my Roon Server
Afterwards the QNAP message was different : User supplied version
But ARC still can’t access to the core located on my QNAP NAS.
I opened another thread (see below) to avoid confusion due to slight configuration differences with this case.