Xamarin Studio won't start (windows 8 64x). What's the matter?

Question

Xamarin

Logs from AppData\Local\XamarinStudio-6.0\Logs:
Xamarin Studio is starting
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.Debugger
INFO [2017-07-15 17:10: 56Z]: Add-in loaded: MonoDevelop.SourceEditor2
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.TextTemplating
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.DesignerSupport
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.Xml
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.AspNet
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.UnitTesting
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.RegexToolkit
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.Refactoring
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.CSharpBinding
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.PackageManagement
INFO [2017-07- 15 17:10:56Z]: Add-in loaded: Xamarin.Ide.Insights
INFO [2017-07-15 17:10:56Z]: Add-in loaded: Xamarin.TestCloud
INFO [2017-07-15 17:10 :56Z]: Add-in loaded: MonoDevelop.Debugger.Soft
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.Xamarin.Ide
INFO [2017-07-15 17:10:56Z ]: Add-in loaded: MonoDevelop.MonoAndroid
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.VBBinding
INFO [2017-07-15 17:10:56Z]: Add-in loaded : MonoDevelop.FSharpBinding
INFO [2017-07-15 17:10:56Z]: Add-in loaded: Xamarin.Forms.Addin
INFO [2017-07-15 17:10:56Z]: Add-in loaded: MonoDevelop.VersionControl
FATAL ERROR [2017-07-15 17:10:56Z]: Failed to start Xamarin Studio
System.TypeInitializationException: Initializer of type 'MonoDevelop .Ide.ImageService" threw an exception. ---> System.MissingMethodException: Method not found: "!!0[] System.Array.Empty()".
in Gtk.IconFactory..ctor()
in MonoDevelop.Ide.ImageService..cctor() in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Ide\ ImageService.cs: line 49
--- End of internal exception stack trace ---
in MonoDevelop.Ide.ImageService.Initialize()
at MonoDevelop.Ide.IdeStartup.Run(MonoDevelopOptions options) at C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Ide\IdeStartup.cs: line 209
FATAL ERROR [2017-07-15 17:10:56Z]: Xamarin Studio failed to start. Some of the assemblies required to run Xamarin Studio (for example gtk-sharp) may not be properly installed in the GAC.
System.MissingMethodException: Method not found: "!!0[] System.Array.Empty()".
Server stack trace:
in Gtk.Dialog..ctor()
in MonoDevelop.Ide.Gui.Dialogs.GtkAlertDialog..ctor(MessageDescription message) in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\ core\MonoDevelop.Ide\MonoDevelop.Ide.Gui.Dialogs\GtkAlertDialog.cs: line 50
in MonoDevelop.Components.Extensions.AlertDialog.RunDefault() in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Components.Extensions\AlertDialog.cs: line 83
in MonoDevelop.Components.Extensions.PlatformDialog`1.Run() in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Components.Extensions\PlatformDialog.cs: line 100
in MonoDevelop.Components.Extensions.AlertDialog.Run() in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Components.Extensions\AlertDialog.cs: line 78
at MonoDevelop.Ide.MessageService.InternalMessageService.GenericAlert(Window parent, MessageDescription message) at C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Ide\MessageService.cs :line 541
in System.Runtime.Remoting.Messaging.Message.Dispatch(Object target)
in System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg)
Exception rethrown at [0]:
in System.Runtime.Remoting.Proxies .RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
in System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
in MonoDevelop.Ide.MessageService.InternalMessageService.GenericAlert(Window parent, MessageDescription message)
in MonoDevelop.Ide.MessageService.GenericAlert(Window parent, String icon, String primaryText, String secondaryText, Int32 defaultButton, CancellationToken cancellationToken, AlertButton[] buttons) in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\ src\core\MonoDevelop.Ide\MonoDevelop.Ide\MessageService.cs:line 450
in MonoDevelop.Ide.MessageService.ShowFatalError(String primaryText, String secondaryText, Exception ex) in C:\data\lanes\3510\0ccfcd52\source\ monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Ide\MessageService.cs:line 187
in MonoDevelop.Ide.IdeStartup.Run(MonoDevelopOptions options) in C:\data\lanes\3510\0ccfcd52\source\monodevelop\ main\src\core\MonoDevelop.Ide\MonoDevelop.Ide\IdeStartup.cs: line 250
in MonoDevelop.Ide.IdeStartup.Main(String[] args, IdeCustomizer customizer) in C:\data\lanes\3510\0ccfcd52\source\monodevelop\main\src\core\MonoDevelop.Ide\MonoDevelop.Ide\IdeStartup.cs :line 594
.