Here is the Microsoft incident:
EX255650, Exchange Online, Last updated: May 11, 2021 1:55 PM
Estimated start time: May 11, 2021 1:54 PM
Issue type: Incident
Manage notifications for this issue: User impact
Users may be unable to view email message content within Outlook.
Latest message, May 11, 2021 1:55 PM
Title: We’re looking into a potential problem affecting email content within Outlook User Impact: Users may be unable to view email message content within Outlook. More info: Initial reports indicate that Outlook on the web is unaffected and users with access to Outlook on the web can view email messages there while we work on a solution. Current status: We’re investigating a potential issue affecting users’ ability to view email message in Outlook. We’ll provide an update in 30 minutes. Scope of impact: This issue could affect any user attempting to view an email message in Outlook.