In order to the bugs MC-53686 (World loads automatically and MC-53651 (World doesn't load) I had to finish the Minecraft task via the Task manager.
Then I wanted to start a new Window from the Launcher which was still open (other task) bot the play button was grey and I had to finish the Launcher too to start Minecraft.
I think the Launcher wanted to receive a "note" from the Game that it closed but because I used the task manager it didn't get it and so thought the Game was still running.