Can you create a startup flow that deletes all the users from the database? :-)
And create a before commit microflow for the account entity so you can give a error when you exceed the 9 users.
This to avoid the mendix bug.
Coincidentally, I just ran into the exact same issue and Cillus' response is the correct answer. An after start-up microflow that deletes some users to bring the total under 10 again will allow you to regain access to the application.
This is, of course, terrible functionality if intended and a pretty bad bug if not. Surely Mendix does not intend for applications, sandbox or no, to simply crash and become unusable as soon as an 11th user is added on a 10 user license?
Adding the 11th user should simply be prevented, along with an appropriate notification.
This is indeed an inconvenient bug. A fix for this will be released next week in 6.4. We will also back port the fix to 5.21.x.