Error opening PortableClassLibrary Project in Xamarin/Windows

I have a project where i have a portable class library created. When i work under macosx all is perfect and working. But when i tried today to open the same project under Windows 8.1 (fresh install of Xamarin 4.2.2 today) i got following errors:

Unknown solution item type: {786C830F-07A1-408B-BD7F-6EE04809D6DB}

Load operation failed. Unknown solution item type: {786C830F-07A1-408B-BD7F-6EE04809D6DB}


MonoDevelop.Projects.Formats.MSBuild.UnknownSolutionItemTypeException: Unknown solution item type: {786C830F-07A1-408B-BD7F-6EE04809D6DB}
   bei MonoDevelop.Projects.Formats.MSBuild.MSBuildProjectHandler.CreateSolutionItem(IProgressMonitor monitor, MSBuildProject p, String fileName, String language, String itemType, Type itemClass)
   bei MonoDevelop.Projects.Formats.MSBuild.MSBuildProjectHandler.Load(IProgressMonitor monitor, String fileName, MSBuildFileFormat expectedFormat, String language, Type itemClass)
   bei MonoDevelop.Projects.Extensions.DotNetProjectNode.LoadSolutionItem(IProgressMonitor monitor, String fileName, MSBuildFileFormat expectedFormat, String itemGuid)
   bei MonoDevelop.Projects.Formats.MSBuild.MSBuildProjectService.LoadItem(IProgressMonitor monitor, String fileName, MSBuildFileFormat expectedFormat, String typeGuid, String itemGuid)
   bei MonoDevelop.Projects.Formats.MSBuild.MSBuildFileFormat.ReadFile(FilePath file, Type expectedType, IProgressMonitor monitor)
   bei MonoDevelop.Projects.ProjectService.ReadFile(IProgressMonitor monitor, String file, Type expectedType, FileFormat& format)
   bei MonoDevelop.Projects.ProjectService.<>c__DisplayClass2.<ReadSolutionItem>b__0(IProgressMonitor param0, String param1)
   bei MonoDevelop.Projects.DefaultProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.CompiledAssemblyExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName)
   bei MonoDevelop.Projects.ProjectServiceExtension.LoadSolutionItem(IProgressMonitor monitor, String fileName, ItemLoadCallback callback)
   bei MonoDevelop.Projects.ProjectService.ReadSolutionItem(IProgressMonitor monitor, String file)
   bei MonoDevelop.Projects.ProjectService.GetWrapperSolution(IProgressMonitor monitor, String filename)
   bei MonoDevelop.Ide.RootWorkspace.BackgroundLoadWorkspace(IProgressMonitor monitor, String filename, Boolean loadPreferences, Boolean reloading)

What can I do about it?

Thank you for any adive.

Kind regards, Hannes