User email is invalid ( address has been not used for any other accounts )

I attempted to change the login account of a certain user.
But, the change process was rejected with the message 'User email is invalid'."

To seek a resolution, I referred to the tips provided in the following link.

User email is invalid – Box Support

My target user had indeed used the same email address for another private account. Therefore, I asked this user to change the login account on private account side.

Afterwards, I attempted to change the login account once again.

However, the result remained unchanged.

I speculated that it might take some time for the changes to be reflected on the server side, so I waited. However, even after one week had passed, the situation remained unchanged.

Could you please provide me with some hints as to why I am unable to change this user's login account?

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