Hey JSchuch,
Can you provide me with a screenshot of your UMG set up so I can reproduce this on my end? Getting an accurate repro case is important for reporting the crash. If you think it would be easier to send me your example project, I am not opposed to testing that on my end.
I am assuming your startup map UMG blueprint is being called to the viewport within the level blueprint, and then you have two separate buttons. One execute the console command Open Level, and the other is a simple Quit?
Let me know if you have further questions or need additional assistance.
Cheers,