Initially it was using a media player texture, but I switched it out, to just be a simple imported png image, no difference.
I did some more testing over the weekend, and it the material is definitely a part of it, using it on clean new objects causes the crash, and swapping out any instance of it stops it.
However, I did find another interesting fact: the crash also stops when one other specific blueprint actor, mainBP, is deleted from the map.
i.e. I have a map with a static mesh actor dragged into it (carrying my material), mainBP, and some other stuff, and the crash will stop if I delete either the static mesh, or mainBP
mainBP is an instance of essentially as a singleton-like BP class that contains the most of level’s script (i.e. what probably should be in the level blueprint, we set it up when we were young and naive), but it shouldn’t be interacting with the material, or the object with it applied, at all. They are new things I just added to the project, and neither one can be seen in the reference viewer for mainBP or vice versa.
I’m going to look through it the next time I get a chance, and see if there’s any specific part of it that leads to the crash, so we’ll see if I find anything