Build Lighting Failing 4.8

Hey guys everytime i try to build lighting I get this error,

[StartJobExecution] Exception was: System.ComponentModel.Win32Exception (0x80004005): The process cannot access the file because it is being used by another process
at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)
at Agent.AgentJob.StartJobExecution() in d:\BuildFarm\buildmachine_++depot+UE4-Releases+4.8\Engine\Source\Programs\UnrealSwarm\Agent\Jobs.cs:line 681

It was working fine, got a lighting build done then this starts happening seemingly randomly.

Any ideas how to fix?