Users may have been unable to load themes in Outlook on the web

Title: Users may have been unable to load themes in Outlook on the web

User impact: Users may have been unable to load themes in Outlook on the web.

Final status: We've received confirmation from a sample of impacted users that the mitigation was successful and the issue is no longer occurring.

Scope of impact: Some users attempting to load themes in Outlook on the web may have been impacted.

Start time: Friday, July 24, 2026, at 11:00 AM UTC

End time: Monday, September 7, 2026, at 8:38 AM UTC

Root cause: A recent update, intended to enhance security hardening, inadvertently introduced a regression which was resulting in impact.

Next steps: - We're reviewing our update procedures to better identify similar issues during our development and testing cycles.

This is the final update for the event.

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've determined that a recent update, intended to enhance security hardening, inadvertently introduced a regression which is resulting in impact. We've reverted the update and following a period of monitoring validation, confirmed that impact has been resolved for the majority of affected users. We're continuing to work with a small number of users who remain affected to conduct additional troubleshooting to fully restore the service.

Scope of impact: Some users attempting to load themes in Outlook on the web may be impacted. This information may be updated as our investigation continues.

Start time: Friday, July 24, 2026, at 11:00 AM UTC

Root cause: A recent update, intended to enhance security hardening, inadvertently introduced a regression which is resulting in impact.

Next update by: Wednesday, September 16, 2026, at 9:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We’re continuing our efforts with a subset of affected users to gather additional details regarding the expected theme configuration as well as if users are able to reproduce the issue so we can determine our next troubleshooting steps.

Scope of impact: Some users attempting to load themes in Outlook on the web may be impacted. This information may be updated as our investigation continues.

Next update by: Monday, September 14, 2026, at 9:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've reviewed diagnostic data from affected users and confirmed that theme data is being retrieved during client startup. We're working with affected organizations to gather additional details regarding the expected theme configuration and are comparing data from impacted and recovered users to identify any missing information or visual elements that may be contributing to the issue.

Scope of impact: Some users attempting to load themes in Outlook on the web may be impacted. This information may be updated as our investigation continues.

Next update by: Friday, September 11, 2026, at 6:30 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've obtained the HAR logs provided by affected users and identified activity indicating theme data is being retrieved during client startup. We're correlating these findings with machine logs from the affected devices to determine whether this behavior is contributing to the reported impact and to isolate the underlying root cause.

Scope of impact: Some users attempting to load themes in Outlook on the web may be impacted. This information may be updated as our investigation continues.

Next update by: Thursday, September 10, 2026, at 11:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We're awaiting a response from your organization with HAR logs from affected users who continue to experience impact, as the issue persists despite the rollback. These logs will help us further investigate the remaining impact.

Scope of impact: Some users attempting to load themes in Outlook on the web may be impacted.

Next update by: Thursday, September 10, 2026, at 9:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We're awaiting new HAR logs from users who continue to experience impact following the rollback to help determine why they're still encountering the issue.

Scope of impact: Some users attempting to load themes in Outlook on the web may be impacted.

Next update by: Tuesday, September 8, 2026, at 9:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've received confirmation from a large portion of affected users that impact is resolved; however, some users are still reporting that impact persists for them. We're working to gather and review new HTTP Archive format (HAR) trace logs from affected users to help understand why impact still persists for them.

Scope of impact: Impact is specific to users attempting to load themes in Outlook on the web. This information may be updated as our investigation continues.

Next update by: Monday, September 7, 2026, at 9:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've received confirmation from some affected users that impact is resolved. We're reaching out to additional impacted users to confirm that the issue is fully resolved.

Scope of impact: Impact is specific to users attempting to load themes in Outlook on the web. This information may be updated as our investigation continues.

Next update by: Friday, September 4, 2026, at 3:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've rolled back a recent authentication related change that we suspect is contributing to the impact. We're reaching out to affected users to confirm whether our mitigative action has resolved this issue, which will also aid in confirming our root cause theory.

Scope of impact: Impact is specific to users attempting to load themes in Outlook on the web. This information may be updated as our investigation continues.

Next update by: Thursday, September 3, 2026, at 9:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: Our investigation of the web startup process issue is still ongoing to further isolate the reason for the impact and help us determine the next steps to remediate the impact.

Scope of impact: Impact is specific to users attempting to load themes in Outlook on the web. This information may be updated as our investigation continues.

Next update by: Thursday, September 3, 2026, at 2:00 AM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We're continuing to review our service side logs to gather more information regarding the web startup process issue and determine why it's preventing the themes from loading, so that we can developed a mitigation strategy.

Scope of impact: Impact is specific to users attempting to load themes in Outlook on the web. This information may be updated as our investigation continues.

Next update by: Wednesday, September 2, 2026, at 10:30 PM UTC

Title: Users may be unable to load themes in Outlook on the web

User impact: Users may be unable to load themes in Outlook on the web.

Current status: We've identified an issue occurring during the Outlook on the web startup process that's preventing themes from loading as expected. We're reviewing system logs to understand what is causing the behavior, which will help us isolate our next steps toward remediation.

Scope of impact: Impact is specific to users attempting to load themes in Outlook on the web. This information may be updated as our investigation continues.

Next update by: Wednesday, September 2, 2026, at 8:30 PM UTC