I got stuck on the loading screen after trying to Launch the parts at this photo bellow. I can hear the game controls and even the tutorial thing showed up, but all the rest of the UI is gone.
All parts used in the vessel at the time of the bug are visible in the image.
1st time it happened I restarted the game.
2nd time it happened used F9 by accident and had the UI back, everything working normal. Then I launched it again with the module at the right, the legs and the stairs, only, and it worked. I was testing if it was the stairs.