Sorry for not offering specifics, but your speculations are pretty on point.
I have an Inventory/Crafting system which both use multiple structs. I also set up Interfaces to try to allow my character to pick up a key and unlocked specific doors. I was unable to load the project at all until I set the map to a default blank map. Whenever I loaded my actual level the engine wouldn’t start.
After I would get into the engine I would get a crash every time I clicked on any blueprints or level. Even if I right clicked to try and delete or duplicate it, it would cause a crash. So I went in and found the most stable backups I could and started switching files wherever I thought the issue was (character blueprints, level blueprints, all of my widgets and interfaces, etc…).
I eventually got everything to work again, but when I closed the project and tried to load it again I kept getting the crash. So I tried verifying my engine version and it wouldn’t work. So I deleted my visual studio 2015 redistributables and everything, but now my engine is gone completely. I’m redownloading now to see if the issue is fixed.
I hope this helps. I’ll add more if needed.