The same obscure thing keeps crashing lighting builds

Main problem:

The same obscure thing (v1.58E06AB8B4CC07117C2393116B3F127BA9D5F0AE.mtrlgz) keeps crshing lighting builds.

Steps tried:

  • I have purged duplicate material names and duplicate mesh names.
  • I have cleared the SwarmCache.
  • I have deleted the DerivedDataCache.
  • I have made sure SwarmAgent isn’t already running before initiating a lighting build from within the Editor.
  • I have tried both the Launcher-based 4.14.3 Editor and the Editor compiled from the Release source (yesterday).
  • I have copied all of the assets into a new Blank Level without lighting.
  • I have added a bit of basic lighting.
  • I have had the Reflection Capture re-capture the scene.
  • I have copied the project to another drive, and done all of the above on it.


Nothing has worked, and it fails on that GUID every time (yes, even after clearing cache, deleting cache contents, copying whole project to another internal drive, running under different Editors.)

SwarmAgent log:

11:40:32 PM: … initialization successful, SwarmAgent now running
11:40:32 PM: [Interface:TryOpenConnection] Local connection established
11:40:32 PM: [Job] Accepted Job 7952B2BB-44ADF6B9-C78A4E93-3CDFE5EF
11:40:48 PM: [Interface:OpenChannel] Error: System.IO.IOException: The process cannot access the file ‘C:\Git\UnrealEngine\Engine\Saved\Swarm\SwarmCache\AgentStagingArea*v1.58E06AB8B4CC07117C2393116B3F127BA9D5F0AE.mtrlgz*’ because it is being used by another process.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.FileStream…ctor(String path, FileMode mode, FileAccess access, FileShare share)
at NSwarm.FSwarmInterface.OpenChannel(String ChannelName, EChannelFlags ChannelFlags)


  • Lightmass building with SwarmAgent starts on a different computer, but that computer never finishes, takes too long or runs out of virtual memory.
  • What more can I try?

I have also made sure that none of the objects have a scale of 0,0,0.

I have also uninstalled all versions of UE4, including the Launcher; then, reinstalled the latest Launcher from Epic, and only the latest Engine (which is the version I have been working in).

I have tried updating the C++ redistributables and the VS redistributables (which would not install, because a newer version is already installed.)

Oh, yeah, and I also tried Verifying the Engine instance via the Launcher … both on the installation that existed before the problem, and also on the re-installed version.

Hey jinngonqui,

Have you been able to reproduce this is a new blank project, or are you testing the same project? This does not mean a new level or new project with the content migrated over. I mean a completely new blank blueprint or C++ project.

Just in case there are things you haven’t tried yet, take a look at the guide below.

Swarm Agent Troubleshooting Guide

Can you provide me with your ‘dxdiag’ so I can take a look at your machines specs?

Make sure your firewall is disabled for Swarm Agent as this can prevent your lighting from building as well.



I have tried it on the same project, in a fresh .umpa scene, with just a few BSD’s, and starter-content materials. Works great. I have been able to delete all of the migrated assets from the troubled .umap, and it works. So it seems clearly an asset in the Content folder, which only bugs things up when it’s in a .umap. I am going to try divide and conquer on bringing back in the migrated assets. I’ll try the dxdiag before I do that: one on the old .umap that has all of the migrated assets, and is failing; and one on the new .umap devoid of them migrated assets.


ran into to this again today(feels like this is a monthly occurence). Went through all of my streaming levels, turning them off one at a time, until it baked properly to find which level contained the issues. After that I deleted 1/2 of the assets, which allowed it to bake properly… So I undid that and deleted half of those assets, until I could narrow it down to one asset that was causing this issue. I deleted that, and went on with my day

Funny(and by that I mean infuriating) enough when I reloaded the level in again with that bad asset after successfully building lights it worked fine… no idea why but yet another reason I have no hair after using this engine for 3 years