Silverlight with unknown error.

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Silverlight

Silverlight5

I am getting a strange error in two of my projects. I have not altered them for some time - so I am very confused by the error. I unchecked the Generate Silverlight Manifest File and now the error is: The "XapPackager" task failed unexpectedly. I have tried to clean the solutions and rebuild, but that fails. Any ideas?

The error:

The "CreateSilverlightAppManifest" task failed unexpectedly.
System.TypeInitializationException: The type initializer for '205519505' threw an exception. ---> System.MissingMethodException: Method not found: 'Void 854179471.2058010023(516896233, Mono.Debugging.Client.StackFrame)'.
   at System.Reflection.RuntimeAssembly.GetType(RuntimeAssembly assembly, String name, Boolean throwOnError, Boolean ignoreCase, ObjectHandleOnStack type)
   at System.Reflection.RuntimeAssembly.GetType(String name, Boolean throwOnError, Boolean ignoreCase)
   at System.Reflection.Assembly.GetType(String name)
   at 1257696219.2037564253(212491372& 1969873877, Assembly 2008445448)
   at 205519505..cctor()
   --- End of inner exception stack trace ---
   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 Microsoft.Silverlight.Build.Tasks.SR..ctor()
   at Microsoft.Silverlight.Build.Tasks.SR.GetLoader()
   at Microsoft.Silverlight.Build.Tasks.SR.GetString(String name, Boolean& fallbackUsed, Object[] args)
   at Microsoft.Silverlight.Build.Tasks.SR.GetString(String name, Object[] args)
   at Microsoft.Silverlight.Build.Tasks.CreateSilverlightAppManifest.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask, Boolean& taskResult)