I have found a real irritating bug that has destroyed my whole project. I used some meshes from the Arbitrary model pack, nothing wrong you would think. Well The meshes somehow sould not work with their materials and got blank when dragging those into the 3d editor.
After deleting the meshes in the editor, plus deleting the files from the content browser the meshes are still named in the WorldOutliner.
I cannot safe the levels anymore because of an error, and after restart the deleted meshes are reapearing in the WorldOutliner, but they are ghosts! See the Computermouse, the desk, etcetera, they all show as if they are still somewhere in the 3d environment.