I have the same thing on all of my Synology Diskstations that manage many peers.
These messages, at the very top of the GUI, came up to me when many peers were connected. But that doesn’t matter, you can ignore it, the directories are still synchronized correctly, in my case.
Test this with a comparison tool so that you are sure.
Looks like you had the SynoCommunity Syncthing package installed on your old Synology; did you install the same one, or did you install the ‘official’ package instead?
Yes - there are two packages. The SynoCommunity one was around for quite a while before the official one was released (see New package for Synology NAS ).
They both use different usernames for the Syncthing process - so if you’re using the new package, it won’t be expecting to use the sc-syncthing username.
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.