Configuration Manager manifest synchronization failing since 8 July 2026

Hi Rajha,

The fact that this started almost simultaneously across three independent ConfigMgr hierarchies strongly suggests this is not a local Configuration Manager issue, but rather a Microsoft service-side or endpoint problem.

Receiving a plain "OK" response instead of a signed ConfigMgr.Update.Manifest.cab explains why SMS_DMP_DOWNLOADER fails the trust validation. It has nothing valid to verify.

I'd recommend:

• Capturing the HTTP response headers and body (using Fiddler or another network trace) to confirm the endpoint is consistently returning only OK.
• Checking whether any proxy, SSL inspection, or firewall device could be modifying the response, although this is unlikely if all three environments are affected.
• Reviewing the Microsoft 365 Admin Center and the Configuration Manager release notes for any recent service advisories.
• Opening a Microsoft support case and providing the dmpdownloader.log, ConfigMgr version, tenant ID, and the raw HTTP response. The fact that multiple independent hierarchies exhibit the same behavior is a strong indicator that Microsoft can use to expedite the investigation.

Please post an update if you hear back from Microsoft.  It would be helpful for others seeing the same manifest synchronization failure.