Failure to renew Realm with error 'The Realm name "<Realm name>" is not allowed'

When trying to renew a Realm subscription I keep getting the error "The Realm name <Realm name> is not allowed."

This is despite this being a nearly 12 month old Realm and using the default realm name, i.e. <gamertag>'s Realm.

The issue is present on both Android (v1.18.32) and Switch (v1.18.32).

All permissions are allowed with my Microsoft/Xbox account such as seeing my profile, seeing my club memberships.

Also, I have tried contacting Support but received no answer after 24 hours (I'm not sure what their typical response time is).

EDIT: Also, this is not a child's account.

EDIT 2:  Customer support replied suggesting allowing all permissions on the Xbox account.  I'm waiting to hear back if they have any other suggestions.