No permission on system folders

Hi

Since quite a while I am trying to sync system folders, but having no luck so far. Syncthing complains with:

hashing: open /var/lib/mopidy/m3u/Diverses.m3u8: permission denied

This is what I want to achieve: I have two server running with Debian Trixie (FreedomBox). On these servers I have Mopidy music server running and with Syncthing I want to sync the playlists located in /var/lib/mopidy/m3u with each other. The files are *.m3u8.

All my attempts did not lead to a success. Mainly I did try:

sudo chmod -vR 777 /var/lib/mopidy/m3u

and

sudo setfacl -Rm u:syncthing:rwX /var/lib/mopidy/m3u
sudo setfacl -dRm u:syncthing:rwX /var/lib/mopidy/m3u

When I use chmod, then Syncthing does one synchronization. The same happens when I use setfacl. After the one sync I receive a permission denied again.

The ignore permissions is ticked. Syncthing doesn’t run in a container.

Some information:

cat /etc/passwd
mopidy:x:109:29::/var/lib/mopidy:/usr/sbin/nologin
syncthing:x:112:113:Syncthing file synchronization server:/var/lib/syncthing:/usr/sbin/nologin
getfacl /var/lib/mopidy/m3u
getfacl: Removing leading '/' from absolute path names
# file: var/lib/mopidy/m3u
# owner: mopidy
# group: audio
user::rwx
group::rwx
other::rwx

and after setfacl:

getfacl /var/lib/mopidy/m3u
getfacl: Removing leading '/' from absolute path names
# file: var/lib/mopidy/m3u
# owner: mopidy
# group: audio
user::rwx
user:syncthing:rwx
group::rwx
mask::rwx
other::rwx
default:user::rwx
default:user:syncthing:rwx
default:group::rwx
default:mask::rwx
default:other::rwx

Both servers are hosting several services and therefore I want to be cautious and not break anything. Similar questions have been asked, but I couldn’t find a solution for me.

This is probably due to one of the hardening options in the systemd unit file.