Using classic Outlook for Windows in business environments
Finally got to talk to someone at MS- this was their reply:
Thank you for the information. Please be advise that Microsoft is aware of this known issue already. This started to happen even last week and we are already working with this matter. We even have created a ticket number for this issue while it is still happening.
The ticket number for the emerging issue is INC31680156.
So they are on it, but no idea when it will be fixed.
[EDIT] They also confirmed that the account was secure, no need to change passwords, etc