I already tried to restart my computer (three times... ) and to reinstall the programm - no use.
I have no idea, which programm uses the catalog or whatever is going wrong there.
Help, please!
Try looking in your catalog folder to see if there is a file with the .lock extension. It should have the same name as your catalog file if one is there. If you find such a file you need to delete it. After you do that Lightroom should start normally.