RESOLVED: Our vendor, Microsoft, reports resolution of the issue that caused some Outlook customers who use the web interface to experience trouble accessing their mailboxes. 

working as expected

Some Outlook customers who use the web interface may be experiencing issues accessing their mailboxes. IT is aware of the issue and investigating. Customers can still access their mailboxes using the desktop or mobile interfaces.

Early diagnosis: We identified that an issue with a recent Outlook on the web build within our service infrastructure is causing some requests to access shared mailboxes to fail. We're developing a fix that will allow the requests to process as expected and remediate impact. Once the fix is ready, we'll validate it before deploying it to the affected infrastructure.

Some Outlook web customers may be experiencing issues