After another test, the use of an internal system user has no advantages, but rather disadvantages:
1.) The assignment of rights for the main directories as a an internal system user is cumbersome, if “syncthing.net” would be a user or a group user, this could be easily and quickly stored in the main directories in the system control.
2.) The same errors also occur in this version of Kastelo. It always happens with the same main directories that messages appear, such as “error while traversing / volume1 / abcxyz: permission denied”.
3.) In directories, such as “photo”, the rights are assigned by default in the photostation and not in the control panel. In contrast, no rights for internal system users can be configured in the photostation, only for users and group users. Only with special knowledge the rights can be assigned in the system control by removing the ACLs in the subfolders. Only a few user can do that (!).
I therefore strongly advise (!) To store “syncthing.net” not as an internal system user, but as a user or group user, this has only advantages.