Hi @Bernard_Forgues ,
This does look like it could be a permissions issue regarding ffmpeg. I would see if you can fix the permissions, or if not, then maybe making a Roon backup and reinstalling RoonServer on the NAS as per https://roononnas.org/en/synology-dsm6/ instructions might be the best way forward. I was also able to locate the below thread which may provide further insights:
https://community.roonlabs.com/t/manually-adding-ffmpeg-does-not-work/177070