Hi varomix,
That’s a known bug. What you’re seeing is the frame window. This usually happens when you drag the Level Editor tab out of it. We have plans to fix this, but it requires some more elaborate changes in how the Editor’s windows are managed, so I can’t give you an ETA yet.
In the meantime, if you are unable to drag the Level Editor window back into the frame, then you can delete the EditorUserSettings.ini file in your project’s /Saved/Config directory.