Great idea! Thank you for your feedback!
I have just checked permissions using file station and found that the permissions for the shared folder didn’t get applied to any folders or files showing in file station. So, I reapplied them there and propagated them using the “apply to this folder, sub-folder and files”.
Really weird! I never had to do so before.
For good measure, I have now restarted syncthing on Synology. Sadly, it failed with the same six objects again. Why does it even want to create objects that no longer exists anywhere?