Earlier this year I got a report about this one, fixed for the next release but thought it should be mentioned here in more detail since someone else came to me with the same issue more recently as there hasn't yet been a release since to include the patch.
A new encounter added in Beta 12 was apparently not compatible with RPGLIKE and outright crashes it if that encounter is generated in the Proximity Caves and you pass your first turn in that map.
The player who first reported it provided a save and I was able to easily find the cause, to be fixed with the next release, but until then if anyone has this issue I can fix your save file and allow you to continue/finish the run if you want! The easiest place to contact me and handle that would probably be the Cogmind Discord.