Outlook and Outlook for Mac users may experience problems this afternoon due to an issue with Microsoft's servers. The symptoms of the issue are that Outlook will pop up a window with a brief error message "Problem." Various activities in Outlook may trigger this message.
Update Sep 05 4:30pm: Microsoft has resolved the outage and Outlook should now be working normally.
Update Sep 05 2:15pm: Microsoft is currently reporting an outage in one of their data centres and this appears to be the source of the problems we are experiencing. Office 365 on the web (portal.office.com) is working and does not appear to be affected by this outage.