Microsoft was able to identify and fix the problem on their end earlier today. The problem was caused by incorrect routing of traffic for Outlook add-ins and calendar free/busy information as result of a recent update Microsoft performed in this region. We have confirmation from most of affected customers that the issue is no longer present in their environments. The official communication about the problem (ID EX744850) was posted by Microsoft in Microsoft 365 Admin Center - it can now be found in Health/Service health/Issue history under the following title: Some users may have been unable to leverage Outlook add-ins or view Calendar free/busy information.
For your reference, here’s the entire message from Microsoft that was posted there:
Microsoft Reference: EX744850
Issue Status: RESOLVED
Issue Description: Some users may have been unable to leverage Outlook add-ins or view Calendar free/busy information.
More Info: Users may have been unable to leverage Outlook add-ins or view Calendar free/busy information.
Final Update: 20/03/2024 09:12:00 AM – Microsoft have successfully deployed the aforementioned change within the affected infrastructure and after a period of monitoring confirmed that issue is no longer occurring.
Scope of impact: Impact was specific to users who were serviced through the affected infrastructure.
Root cause: A recent update exposed a code regression that was leading to incorrectly rerouted traffic for Outlook add-ins and calendar free/busy information requests.
Next steps: Microsoft are reviewing their update procedures to better identify similar issues during their development and testing cycles.