I found it !
Okay, so it turns out it actually does not have anything to do with kryonet, but it just crashes while doing some kryonet stuff in the background, or something like that.
It turns out that it was an index out of bounds in my "unresolved moves" list that I use for client side prediction. This list is one of the things that is checked up against when I recieve packets from the server, and the crash turned out to be trivial.
I ended up pinpointing it by running the game with the play button instead of the debug button, this game me a much more useful exception that lead me to find the crash reason.
Thanks for the help anyway, I love the response time on this forum