Microsoft 365 Alert – Service Degradation – Microsoft 365 Copilot Chat – Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error – RESOLVED

Announcements Shagufta Naaz

05/08/2026 08:31:00 AM

NHS.net Connect Reference: INC46900485

Microsoft Reference: CP1447620

Issue Status: RESOLVED

Issue Description: Users may be unable to access Microsoft Copilot Chat in the browser and may encounter a 400 Request Header error.

More info: As a temporary workaround, affected users may be able to restore access to Microsoft Copilot by clearing their browser cookies and then trying again.

Final Update: 06/08/2026 11:22:00 AM – Microsoft has completed deployment of our fix to address the browser authentication information misconfiguration and validated after completing an internal monitoring period that our actions successfully remediated impact. If users are still experiencing impact, we advise them to clear their browser cookies and reload Copilot.

Scope of impact: Impact is specific to some users attempting to access Copilot in the browser.

Root cause: A recent service change that affects the way browser authentication information is stored for Copilot. This can cause some browser requests to exceed expected limits, which may prevent users from accessing the service.

Next steps : Microsoft is analysing the offending service change to isolate why the potential for impact wasn’t caught prior to the update being deployed to affected users. This analysis will help us to improve their update implementation process moving forward to ensure similar impact scenarios don’t occur in the future.