How to fix "Lighting Build Failed. Swarm Failed to Kick off" error?

I’m busy building a few levels in ue4. When I go to build the light though, one level (and only this one) says, “Kicking off Swarm,” followed by, “Lighting Build Failed. Swarm Failed to Kick off.” I have looked through many other questions asked about this and have tried several solutions given, to no avail.

After checking the Swarm log, here is what I have found: 2:26:15 PM: [Interface:OpenChannel] Error: System.IO.IOException: The process cannot access the file ‘C:\Users\greenleaf\AppData\Local\UnrealEngine\4.11\Saved\Swarm\SwarmCache\AgentStagingArea\8487AD2E5DBA413C81373D29B2915895.60A4425F4961A872371649A2B6F92B90.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) in i:\Perforce2\UE4-Orion\Engine\Source\Editor\SwarmInterface\DotNET\SwarmInterface.cs:line 1604

What exactly is the problem here? And how do I fix this?