Could not load type 'ch.sycoforge.Unity.Editor.EditorWindowExtension'

If the following exception occures while updating to a new version

Unhandled Exception: System.TypeLoadException: Could not load type 'ch.sycoforge.Unity.Editor.EditorWindowExtension' from assembly 'ch.sycoforge.Unity.Editor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'

Unity most likely didn't recreate the asset database. To resolve this issue try reimporting the asset first, if the error still shows up, delete the complete asset and reimport it again.