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This "Bug" or whatever it is, happened once again to me, luckily my last backup was 2 hours ago, but it's still annoying not knowing whether you will be able to open your shader ever again once you close the shaderforge window, though...!
Whenever i try to open the shader, the shaderforge windows just stays grey and I get the following:

NullReferenceException: Object reference not set to an instance of an object
ShaderForge.SF_Node.GetInputData (System.String id)
ShaderForge.SFN_Lerp.IsUniformOutput ()
ShaderForge.SFN_If.IsUniformOutput ()
ShaderForge.SFN_If.IsUniformOutput ()
ShaderForge.SF_NodePreview.Combine ()
ShaderForge.SF_Node.RefreshValue (Int32 ia, Int32 ib)
ShaderForge.SF_Node_Arithmetic.RefreshValue ()
ShaderForge.SF_Node.OnUpdateNode (NodeUpdateType updType, Boolean cascade)
ShaderForge.SF_NodeConnector.SetValueType (ValueType vt)
ShaderForge.SFNCG_Arithmetic.SetOutputValueType (ValueType vt)
ShaderForge.SFNCG_Arithmetic.Refresh ()
ShaderForge.SF_NodeConnector.LinkTo (ShaderForge.SF_NodeConnector other, LinkingMethod linkMethod, Boolean registerUndo)
ShaderForge.SF_Link.Establish (ShaderForge.SF_Editor editor, LinkingMethod linkMethod)
ShaderForge.SF_Parser.LoadFromNodeData (System.String data, Single version, System.String& missingNode)
ShaderForge.SF_Parser.ParseNodeDataFromShader (ShaderForge.SF_Editor editor, UnityEngine.Shader s)
ShaderForge.SF_Editor.InitializeInstance (UnityEngine.Shader initShader)
ShaderForge.SF_Editor.Init (UnityEngine.Shader initShader)
ShaderForgeMaterialInspector.OnInspectorGUI () (at Assets/ShaderForge/Editor/InternalResources/Inspectors/ShaderForgeMaterialInspector.cs:85)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1235)

Warning said: "Your shader might not render properly on your workstation - you need to have Direct3D 11 enabled when working in DX11 mode on Windows. Click the icon to enable Direct3D 11!" So not really helpful for me... :D Quality/Player settings didn't help much either.

But I found a way to overcome my problem:
I just duplicated the ".shader"-file in my explorer folder and also created a new material using that duplicated shader. For some reasons the new material finally displays in the game window.
The old shader still won't display though.

Don't know if it's a Unity or Shaderforge Bug, I leave this info here anyway :D
The previewed Mesh is just a Triangle shaped out of quads.
Have found another way to achieve my goal, still didn't figure out why it wasn't working the way shown above.