.DS_Store should generally be in the ignore list, as that is only metadata from the Synology OS.
It seems to me, that the user which is running Syncthing, doesn’t have write permissions. Did you add the user to the admin group, so it can access and write all folders in the volume(s)?
I always had the errors, and never found a proper solution (probably change who owned and ran the syncthing task, etc) but found an ownership modifier script and ran that on all files at that point - clearly an instant but not permanent solution! Oh, and I’ve lost the script, but would prefer to fix it properly!
Yes, I think you’re right re admin group.
I don’t know which user is running Syncthing or how to find out.
I can see no user that has a name relating to Syncthing.
I can see 4 groups - administrators, gtpp, sc-syncthing and users.
In the administrators group there is just myself, admin and plex.
In fact, I’ve just gone onto the top level share folder and applied the permissions again to all subfolders and I can see the Out of Sync Items, instead of sat at the same high number for weeks, is now decreasing at a pace.
So it seems that the permissions are not being set correctly or something when the folders are created? I dunno!