Hi,
There are so many reasons could cause “Exchange: An unknown error has occurred. Refer to correlation ID” error message.
However, based on your description, it could be some of Exchange attributes has incorrect value. You could run Get-Mailbox, Get-User against the object and view the output via PowerShell.
Then you could try to reset the password on the account in AD, let it sync with O365 and tried again.
Also, please double check Exchange Online license is enabled for the mailbox.
If you have any questions, please feel free to contact me.