Unity 2018.1f1 - editor issues using .NET 4.x

Unity Engine
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Hi, i am not able to always run latest 2018.1 with .NET 4.x loaded. Getting this error message in the beginning then it clears somehow and after a while shows up again and editor stop responding …
How good is it the 4.x support currently ?
Am i suppose to switch to and use it in a project or it is not yet fully supported and better to stick to C# 3.5 !?

ReflectionTypeLoadException: Exception of type 'System.Reflection.ReflectionTypeLoadException' was thrown.
System.Reflection.Assembly.GetTypes () (at <e1a80661d61443feb3dbdaac88eeb776>:0)
UnityEngine.Experimental.UIElements.Factories.DiscoverFactories () (at C:/buildslave/unity/build/Modules/UIElements/UXML/VisualElementFactory.cs:58)
UnityEngine.Experimental.UIElements.Factories.RegisterFactory (System.String fullTypeName, System.Func`3[T1,T2,TResult] factory) (at C:/buildslave/unity/build/Modules/UIElements/UXML/VisualElementFactory.cs:31)
UnityEngine.Experimental.UIElements.Factories.RegisterFactory[T] (System.Func`3[T1,T2,TResult] factory) (at C:/buildslave/unity/build/Modules/UIElements/UXML/VisualElementFactory.cs:37)
UnityEditor.Experimental.UIElements.UXMLEditorFactories.RegisterAll () (at C:/buildslave/unity/build/Editor/Mono/UIElements/Controls/UXMLEditorFactories.cs:18)
UnityEditor.GUIView.get_panel () (at C:/buildslave/unity/build/Editor/Mono/GUIView.cs:42)
UnityEditor.GUIView.get_visualTree () (at C:/buildslave/unity/build/Editor/Mono/GUIView.cs:56)
UnityEditor.GUIView.OnEnable () (at C:/buildslave/unity/build/Editor/Mono/GUIView.cs:170)
UnityEditor.AppStatusBar.OnEnable () (at C:/buildslave/unity/build/Editor/Mono/GUI/AppStatusBar.cs:16)

EDIT : after switching back to .NET 3.5 the project does not want to load anymore !!!

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