Well, for quite a while accessing reminders stored on my Synology NAS from my Mac was broken. Syncing with my iOS devices was no problem but on my Mac I could not see the folders and reminder items. Looking around in the internet showed that it was apparently problem since macOS 10.15. It seems that the Reminders client on the Mac does not accept the LetsEncrypt certificate on the Synology NAS.
Good news: With macOS 13 (Ventura) the sync is back on the Mac. No problems anymore neither on my Intel Mac nor on my Apple Silicon Mac.