URGENT Unable to add existing user as Managed User — "Cannot add this user" error

We have an Enterprise plan with 4 seats purchased. 3 of 4 seats are currently occupied, and 1 seat remains available.

We are trying to add a 4th user as a Managed User, but this user previously had an existing personal Box account associated with their email address.

Steps we have taken so far:

1. Asked the user to delete their previous Box account to free up the email address.
2. Attempted to add the user via Admin Console > Users and Groups > Add Users Manually.
3. Received the initial error ("1 user failed to be added") and clicked Save a second time as recommended in Box documentation.
4. On the second Save, the system shows a continuous loading spinner, and a notification appears on screen: "Cannot add this user."
5. The loading never completes and the user is never added.

What we are requesting:

We need immediate assistance to resolve this issue so that we can add this user to our enterprise as a Managed User. We have been unable to use 1 of our 4 purchased seats due to this problem.

Please take whatever action is needed on your end (e.g., clearing the previous account association for this email address) to allow us to add this user.

We would appreciate support in Japanese if available, but given the urgency, English is also fine.
 

Note: We have previously submitted a support ticket regarding this issue but have not received a response. This is an urgent request - please response asap.