It might be an internal error which causes the mod not to load fully. Would you be able to send me a copy of your data folder from your game's directory to see what the issue is?
Alright so I did a little experimenting and it definitely seems to be related to custom art assets afterall. If I try to assign custom backgrounds or busts, the level breaks in the editor in various ways. The more assets I use the more it breaks; if I just try to load a custom bust, I get the HUD (despite disabling it) but the level otherwise works. If I try to load a custom bust and a custom background, the entire level breaks down like in the sample image.
I forgot to, thank you. But some other weirdness is acting up now; if I assign the (now converted) assets, the game locks up during launch and crashes. I was able to move the assets to a clean copy of the mod tool and even got them to boot in game and work properly by applying them to one of your tutorial levels, so it can't be the assets themselves anymore...
I must have something fucky going on, so I'm going to bring over elements of my game crashing folder to the currently working one until I find what's causing it to break. If I find out I'll let you know. Cheers.