Preview starts but playhead never advances (AE 26.2.1 & 26.3, Windows 11 25H2 Build 26200)

After Effects Preview is completely non-functional.

Environment

• HP Z6 G4 Workstation
• Intel Xeon Gold 6128
• 64 GB RAM
• NVIDIA Quadro RTX 5000 (15 GB)
• Windows 11 Pro for Workstations 25H2
• OS Build 26200.8655

Tested versions

• After Effects 26.3 Build 87
• After Effects 26.2.1 Build 2

Problem

Pressing

• Spacebar
• Numeric Keypad 0
• Preview Play button

changes the Preview button from ▶ to ■ and it remains in the Stop state forever.

The playhead never advances.

What works

• Keyframes work correctly.
• Dragging the playhead manually updates the animation correctly.
• Rendering works correctly.
• RAM cache (green cache bar) is generated.
• GPU acceleration works.
• DirectX diagnostics report no problems.
• System Compatibility Report reports no problems.

What has already been tested

• Reinstalled After Effects
• Installed previous version (26.2.1)
• Safe Mode
• Disabled all third-party plugins
• Reset workspace
• Reset preferences
• Updated NVIDIA Studio driver
• New empty project
• Single PNG test
• Different audio output devices

Nothing changed.

Log output shows

AudioRendererImpl Init starting up...
VideoRendererImpl Init starting up...
EGG RamPreview:
starting preview
successfully started 1 previews simultaneously

No errors are reported.

It appears that After Effects believes Preview has started successfully, but the playhead never advances.

After Effects Log and DxDiag are available if needed.

Manual scrubbing works correctly.

Dragging the playhead manually updates the composition correctly.

Only real-time playback is broken.

I am available to test beta builds or collect additional diagnostic logs if needed.