According to your description, this error message suggests that the user ID you are trying to assign or use is already associated with an existing user in your directory service.
Have you already migrated this mailbox to Exchange Online?
You can use the command Get-Mailbox -Identity fae7c315-3a99-49a7-95cc-84df9d00e8db |fl and Get-Mailbox -identity " Mailbox alias"| FL
If possible, screenshots can be provided for my next troubleshooting.
Also, for mailbox $2TAM41-BE13M28DOQ7T@****.onmicrosoft.com ,are there any special characters in the mailbox? Are there similar errors in other mailboxes with special characters?