4.15 Light Build stuck on exporting to local cache

Hi all,

Just upgraded my scene to 4.15 from 4.14. Whenever I go to build the scene it hangs on export to local cache forever, in the bottom right of the editor it says it is complete but when i flip to the swarm agent it is continuing without ever switching to the next phases so i cannot build my lighting.

I an noticing this in the log

2:56:22 PM: [Interface:OpenChannel] Error: System.IO.IOException: The process cannot access the file ‘C:\Program Files (x86)\Epic Games\4.14\Engine\Binaries\DotNET\SwarmCache\AgentStagingArea\v1.7E0E2027E4CD375CAF7BB15DAD2D177EDCF4FC45.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)