.dll not found exception on execution server

Hi,

I submit a test list to scheduling server, and get following exception:

----------------------------------------------------
Overall Result: Fail
System.Exception: Exception loading assembly with path C:\Program Files (x86)\comparis.ch\TestLibraries\Comparis.Framework2.UnitTesting.dll. ---> System.IO.FileNotFoundException: Could not load file or assembly 'Comparis.Framework2.UnitTesting, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.
   at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
   at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection, Boolean suppressSecurityChecks)
   at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection)
   at System.Reflection.Assembly.Load(String assemblyString)
   at System.UnitySerializationHolder.GetRealObject(StreamingContext context)


   at System.AppDomain.Load(AssemblyName assemblyRef)
   at ArtOfTest.WebAii.Design.Execution.TestAppLoader.LoadRefAssemblies(IList`1 references)
   --- End of inner exception stack trace ---
   at ArtOfTest.WebAii.Design.Execution.TestAppLoader.LoadRefAssemblies(IList`1 references)
   at ArtOfTest.WebAii.Design.Execution.ExecutionEngine.ExecuteTest(ExecuteTestCommand command)
------------------------------------------------------------
'16.02.2012 09:28:07' - Overall Result: Fail
'16.02.2012 09:28:07' - Duration: [0 min: 0 sec: 62 msec]
------------------------------------------------------------

Indeed mentioned .dll is there in the right path and scheduling was worked before. After getting this exception, whenever I submit a new test list to scheduling server, it gives following error in scheduling logs:
----------------------------------------
Machine is busy executing other tests!
----------------------------------------

Unless I reboot the server, I get the above message and after rebooting the server, I get first exception and then above message again afterwards.

Do you have any idea what could be the reason of not finding .dll in that path although it is there.

Thanks

Ercan