Yep! It was the save version being old that was the cause. After renaming the save folders, the current build launches.
Version 39 from itch says v0.10.3 in the corner. Strangely, the game recreates both folders (although "Axol Studio, LLC" is empty).
But looking at my progress it wouldn't be worth the effort trying to figure out what went wrong with the save.
I did encounter a different problem with the latest version while playing my first game from a fresh save. After I got stoneskin from a chest , the game turned solid green and closed. My progress wasn't saved. It says I have 0 soulstones after loading the game.
I would suggest saving your updated soulstones whenever you pick some up (or anything else important), to avoid lost progress in case of a crash.
Am I remembering wrong that you used to be able to aim manually where your weapon would attack by using the second analog stick?
Maybe I am. I would suggest having manual aim as an option. After playing some other survior-likes which have that option it's hard to go back to one without it.
Edit: I got another bug after picking up an item which increased xp or loot (I don't remember which) by 100% and reduced pickup range.
I noticed after that enemies stopped dropping xp entirely. There were literally no more xp drops showing up at all for several minutes. I didn't have any upgrades which increase pickup range. I don't know if that was a factor.
In the end all I could do is let myself be killed because it was impossible to level up.