Some users may be unable to access their Exchange Online mailbox using Outlook on the web (M365)

From M365 Service Health Dashboard:

EX610644

Title: Some users in North America may be unable to access their Exchange Online mailbox using Outlook on the web

User impact: Users in North America may be unable to access their Exchange Online mailbox using Outlook on the web.

More info: Affected users may encounter an unexpected 500 error when accessing Outlook on the web.