As you use ROCK, have you installed the codecs? You can check this by going to the ROCK’s web page. If it’s written somewhere there that it’s missing codecs, then you have to install them (see Roon OS Missing Codecs).
Yes, I did that on my last attempt, and just checked it. The file ffmpeg-git-20200324-amd64-static is in \ROCK\Data\Codecs. I just rebooted the NUC (in case I didn’t do it last time).
After the restart, I went to the System Status page where it reports under Roon Server Software that my installation of Roon is “Missing Codecs”.
That is where I stopped last time as well as this attempt now.