We had several clients call in yesterday telling us that their Outlook application was crashing when it was launched. It appears from Microsoft's post that it was caused by a forced update to the Outlook client. See information provided by Microsoft below:
Title: Users experiencing Outlook connection issues and crashes User Impact: Users may have experienced crashes or may have been unable to access Exchange Online via Outlook.
More info: Users may need to restart their Outlook client for the fix to take effect.
Our analysis indicates that Outlook on the web and mobile clients were unaffected. Users may have been able to leverage those protocols as an alternative means to access email and service features while we remediated this problem.
Affected users would have received an error message that stated: "Outlook couldn't start last time. Safe mode could help you troubleshoot the problem, but some features might not be available in this mode. Do you want to start in Safe Mode?"
Further information on what admins were able to see in event viewer for this crash event could have been found here: https://support.microsoft.com/en-us/office/active-investigation-into-outlook-crashing-on-launch-9c59ad4b-813c-432a-afdc-f14717a4528d?ui=en-us&rs=en-us&ad=us
Final status: We've confirmed that the issue has been successfully resolved after extended monitoring of our telemetry. If users are continuing to see impact, please restart your Outlook client for the changes to take effect.
Scope of impact: This issue may have potentially affected any of your users attempting to use Outlook.
Start time: Wednesday, July 15, 2020, 11:12 AM (3:12 PM UTC)
End time: Wednesday, July 15, 2020, 6:35 PM (10:35 PM UTC)
Root cause: A recent change to the Outlook client inadvertently resulted in this issue.
Next steps:
We'll publish a post-incident report within five business days.