Can't create new user because Load Balancing failed to find a valid mailbox
I’ve included a screenshot of the error.
The admin user in question also can’t manually select a different mailboxdatabase.
SuperAdmin can see load balancing and apply these changes manually.
Am I missing an important setting somewhere?
The load balancing actually occurs on Exchange and this error message is from your Exchange environment. Usually this occurs if you exclude all the databases from provisioning or there is an issue. Chances are if you manually create a user/mailbox in /ECP and don’t select a database it will throw the same error, but if you specify the mailbox database then it would create it. You would need to check and make sure all your databases are not assigned to other companies and/or the IsExcludedFromProvisioning is not set to True on all databases.