I am having the exact same issue. "Error loading next map. Returned to main screen to prevent file corruption."
This is on an Iron Trials run, I am pretty frustrated that my ONLY save seems to not work.
I managed to get the game working again by closing out and verifying file integrity with Steam. Not sure what (if anything) that did, but afterwards there was an unusually long loading screen, an unusually long black screen, and a couple second of stuttering before finally loading up and playing fine. This was actually a huge relief compared to the error message I was getting a couple hours ago.
But the relief didn't last long. Loaded in, sailed around (less than two minutes), got into a nautical battle, almost died, quit out, tried to load back in, same error message.
same exact scenario happening twice in a row. I thought that maybe it was something obscure having to do with the iron trials things, but the guy who posted before me has more than one save (clearly not the iron trials).
As a note (maybe it helps), but the guys that i ran into seemed like the EXACT same vessel that attacked me the first time the error happened. cant remember the name of their captain, but It was the same extreme difficulty, same purple flag, same number of guys, they used all the same tactics and spells, they boarded me as fast as they could, and my whole team was almost wiped out in under 60 seconds. I had not had an encounter like that so far in the whole game (I am only level 5 or 6, but i am very thorough, been playing for like 20 hours) and this one seemed identical. So maybe it has to do with that specific encounter? Devs, please help!
I will try to get it running again and test to see if re-saving after getting it running fixes the problem. Will post results afterwards.