This has been resolved, and reportedly affected mailboxes are successfully logged in at this time.
Update 05/15/2020 17:38 PM 25 days
We are currently investigating an error some mailboxes are giving at login:
Your mailbox appears to be unavailable. Try to access it again in 10 seconds. If you see this error again, contact your helpdesk.
Users can be directed to use LiveArchive as a workaround in the meantime, by signing into their account here: https://admin.exchangedefender.com/livearchive.php
Thank you for your patience and understanding during this process.