We are aware of an intermittent issue impacting users trying to access their mail via OWA, on any browser, which began at October 1, 2020 5:52 AM. This issue is caused by a Microsoft service degradation and effects access to Exchange Online using multiple protocols. Once the downstream issue has been resolved, we expect to see this impact start to remediate immediately.
UPDATE: The service has recovered for the majority of users. We are performing traffic-management optimization measures to ensure the service recovers for all users. Guidance at this time is to refresh your browser, several times if necessary, until your connection to OWA is restored.