This seems like an issue with going out of the bounds of one of your arrays. You mention “Second time no problem.”. Does this mean that you’re no longer getting this crash? If so, unless you’re able to reproduce this crash, there isn’t much we can do. If you do get the crash to occur multiple times, please let me know and I’ll be happy to look into the problem. It will require looking at your code however as it seems like it would be related to your project rather than an issue with the engine.
The constructors of your blueprints are run whenever the map is loaded, otherwise any blueprints placed in your level would never even appear until you hit play. There are a number of things that could cause this. If you’re no longer experiencing this issue with 4.12.5, I’ll be marking this as resolved as it seems that it is.