Update October 16, 2026: For more information related to installation of October 2026 updates for Windows Server and the possible impact on Exchange Server, please see Exchange servers may become uns...
Updated Oct 16, 2026
Version 7.0
Blog Post
We can confirm a matching incident in our Exchange Server SE environment running Windows Server 2022.
- Windows update KB5126655 installed on October 13, 2026.
- First documented MapiExceptionMdbOffline (ec=1142) on October 14 at 19:06 CEST.
- All four mailbox databases remained mounted, and the Exchange Information Store service was running, but mailbox access through MAPI/EWS failed.
- The problem persisted overnight.
- A manual server reboot on the morning of October 15 restored normal operation.
We also observed increased database disk read latency during a concurrent backup operation, so we cannot conclusively attribute the initial failure to the Windows update alone.
The symptoms, error code and timing closely match the incidents described here.
We have not uninstalled KB5126655. Is there a confirmed mitigation or further diagnostic information Microsoft would like us to collect?