Can't add an additional account to a Business or Enterprise?

For some months, we have been seeing sporadic reports of users who can't add profiles they own to their paid tier 2 and 3 accounts. Each time we test this, it's been working for us, until today!
This means finally we have something the engineers can dig into. If we can't resolve this immediately, we will halt billing on all affected accounts, until we do.

Apologies for the inconvenience!