Microsoft側障害の為、復旧までお待ちください。
追記:4/21 16:17 復旧しました。
-------------- Microsoft 回答 --------------
Exchange Online service alert
Incident information
Title: Can't access mailbox via Outlook on the webID: EX209929Start time: Monday, April 20, 2020
Status
Service Restored
User impact
Users may have experienced 500 errors when attempting to access their mailbox via Outlook on the web.
Thank you,The Microsoft 365 team