I'll keep the long story short - I put another project of mine through ENIGMA's ringer and found ENIGMA spits this error and breaks when given GM's Asynchronous events.
I assume since Pre-Draw, Post-Draw and System, along with the application_surface changes aren't implemented in ENIGMA, and because of the funny way they show up in LateralGM ("!EVENT7._75!" for the System event) they get parsed in a way that leads to a crash and burn.
I've uploaded a Hello World of sorts that includes two of the events I use, System and Pre-Draw - I didn't include Post-Draw but I assume that would exhibit the same behaviour. I should also note that the Progress Bar got as far as "Performing Secondary Parsing and Writing Globals" if that's any help.
Good news though is that while the Play/Debug buttons grey out afterwards LateralGM seems to keep running in one piece and I can close it normally or look at the code (I forgot to test saving).