Have you tried a good old fashioned switch off and on again? (Rather than the reset button?)
Yes, thank you. But, no success... so I went for the full reset. O my, was that a learning curve.
It seems as though I cobble together various features over the months and years, my "ho to" documentation is lacking, so I've had to refigure quite a bit out. Fortunately...
1. I have current HB backups of all possible apps. So that went well until I tried to restore the DSM configuration, to discover how to relink old backups, and then to use the original "admin" account to complete the configuration restore.
2. Plex reinstall went well, requiring me to set Plex user rights to my media.
3. Docker (not officially supported) was the real surprise, as all my container seemed to automagically appear... and work!
But I was flummoxed with CardDAV for a while, until I discovered that a reverse proxy I used was the key to my previous working URL... I also discovered that that reverse proxy was responsible for blocking Photo Station launching... I never really understood why PS was so finicky, until I noticed that it was working fine after the reset... When I added back the reverse proxy for the DSM... it killed PS launch, but notably "fixed" my CardDAV set up.
So now... 6 hours later... I believe most everything is back up.
Next to SSH in and see what mess is lurking... that brought this system to its feet early this morning.