Are you using custom tick intervals by any chances? I’m experiencing the same problem with 4.11 or 4.12 (4.10 works fine).
I’ve been able to find the source of crashes - they happen when any of my actors uses non-default tick values. When I reset tick intervals to default, I can migrate to 4.12 no problem. As soon as I set my previous custom values (or any value) in those blueprints, my level crashes during play and next time I launch my project related blueprints crash immediately on opening.