KB5063877: The Update You Need for Stable Windows Clusters

August cumulative update KB5063877 for Windows Server 2019 resolves a July patch flaw that crashed Cluster service and forced repeated VM reboots—especially on BitLocker-protected CSVs.

Microsoft has shipped an August 2025 cumulative update that fixes a Windows Server 2019 bug causing Failover Cluster instability and virtual machines to restart repeatedly after July’s security updates. Admins are advised to install the servicing stack update first, then deploy KB5063877 via Windows Update, Catalog, or WSUS.

REDMOND, Wash. — Aug. 14, 2025 — Microsoft has released a fix for a Windows Server 2019 regression that triggered Failover Cluster outages and repeated virtual machine restarts following July’s Patch Tuesday, restoring stability for organizations running Hyper-V and other clustered workloads. BleepingComputer

The company said the flaw—introduced in the July 8 cumulative update KB5062557—could cause the Cluster service to stop and start in a loop, preventing nodes from rejoining or pushing them into quarantine, with VMs restarting multiple times. The issue was most visible on clusters using BitLocker with Cluster Shared Volumes (CSV). Microsoft now lists the problem as resolved by the August 12 update KB5063877 (OS build 17763.7678). Microsoft Support+1

Administrators should apply the latest servicing stack update (SSU) before installing KB5063877, then deploy the cumulative update via Windows Update, the Microsoft Update Catalog, or WSUS. Microsoft had previously advised affected customers to contact business support for mitigations while the engineering fix was finalized. BleepingComputer+1

What broke and why it mattered
Failover Clustering underpins live migration, role ownership and high availability for Hyper-V and Storage Spaces Direct. When the Cluster service flaps, nodes can enter quarantine and clustered roles—including VMs—restart, producing cascading disruption across compute and storage layers. Event logs on impacted systems frequently showed Event ID 7031 as the service crashed and restarted. NinjaOneManageEngine

What’s fixed
KB5063877 updates Windows Server 2019 to build 17763.7678 and addresses the Cluster service fault path tied to the July update, with Microsoft’s release notes explicitly crediting the August update as the resolution to the known issue. Microsoft Support+1

What admins should do now
Microsoft and patch-management advisories recommend installing the SSU, deploying KB5063877 across Server 2019 clusters, clearing any temporary mitigations, and validating cluster health. Shops using WSUS should approve the update and confirm synchronization, then monitor nodes for stable membership and error-free Cluster service status after reboot. BleepingComputer

The bigger picture
The episode follows a series of Windows Server reliability fixes this year and underscores the need for pre-production rings, canary hosts and automated rollback—especially on high-availability stacks where regressions can ripple quickly through production. BleepingComputer’s coverage notes the fix arrives amid a busy August Patch Tuesday cycle. BleepingComputer+1

Sources:

• BleepingComputer — “Microsoft fixes Windows Server bug causing cluster, VM issues” (Aug. 14, 2025). BleepingComputer
• Microsoft Support — “July 8, 2025—KB5062557 (OS Build 17763.7558)” (Known issue; resolved by KB5063877). Microsoft Support
• Microsoft Support — “August 12, 2025—KB5063877 (OS Build 17763.7678)” (fix release notes). Microsoft Support
• Microsoft Q&A / community reports describing Cluster service restarts and Event ID 7031 in affected Server 2019 clusters. NinjaOne