We are seeing a small amount of customers/users who when accessing a second mailbox in their outlook, get redirected to the 365 portal and asked for credentials.
The issue started 15/10/25 and is still continuing.
As the issue also happens when just in the portal, we suspect a change or error at Microsoft’s end.
No doubt they will be aware of the issue.
A temporary solution is to close and reopen Outlook when it starts to happen.
Note – This only gives a slight respite from the issue, as it comes back after Outlook has been open for a while.
Will update here when we know more.