RE: Maximum registration capacity
Hi Francesca,
With Allow registrants to create their own agenda option was set to No,
sessions should not be constrained by maximum capacity field as normal situation.
I tested with information you've posted in two environments (9.1.0.9234 with wave 2 release not enabled and wave 2 release enabled) and didn't encounter your issue.
The only situation that I could reproduce your issue is that I dragged maximum registration capacity field to session main form(in advanced settings > customization) and set a small value manually,

(maximus was set to 1 and 1 contact added for its registration manually)
then the session will show an error notification: session has reached maximum capacity..
and then I also got error as yours:

Suggestion: before finally raise a ticket for help, you could try to set a large number for session registration manually in form editor then test whether issue could be fixed.
If above steps not work, you may raise a ticket for support because the feature shall work as expected with the option be set to No:
https://admin.dynamics.com/?referer=mbssupport
Regards,
Clofly