The problem has to do with Synology’s partly strange ACL system. In my opinion this have nothing to doe directly with Syncthing. However, in most cases I can remove these messages.
In such cases, I’ve deleted the Synology ACLs by setting the owner and group to
root and octal rights to
0777 for all files and subdirectories in the affected peers. Then I go back to Synology DSM into filestation and inherit the permissions again in these peers in all files and subdirectories.
This is kind of a standard procedure if I have any kind of permission issue. But I outline, is also a procedure more for experts and people, which know the background of such things.