Swarm agent failed to kick off, common solutions don't work

Development Rendering
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Hi there.

I have some serious issues with Swarm Agent. I’m using engine version 4.16.3.
Building lighting always fails at 67% of the “Exporting lighting data” process with the following message: "Lighting build failed, Swarm agent failed to kick off.
The swarm agent log leaves this message at 50% of the process:

Error: System.IO.IOException: The process cannot access the file 'C:\Lightmass\AgentStagingArea\v1.B3E64758A3E2C4852BD915EB036C21DA0C151FF5.mtrlgz' because it is being used by another process.

The swarm agent graph keeps moving forward even after both errors (As “Exporting Scene”) until I force close Swarm Agent or start a new build.
Here’s what I tried to solve the issue, with no success:

  1. Downloaded the engine from GitHub, doesn’t fix it. Tried rebuilding UnrealLightmass.cpp, doesn’t fix it.
  2. Checked for duplicate meshes, materials and material instances. Renamed or deleted the ones I found. I did this multiple times (on >2000 items).
  3. Made a new empty project and migrated the map to it.
  4. Tried to delete meshes from the map. This worked after deleting several meshes (in fact the building worked even with the “corrupted” meshes after I opened and closed the engine), but only for a while. At some point, the error returns even if I delete the suspected meshes again.
  5. Cleared and validated swarm cache, multiple times.
  6. Reinstalled the engine through the launcher.
  7. Deleted the project’s Intermediate and Saved folders.

I’m completely dumbfounded. Any suggestion will be tested. Thank you!