Hi there, I am also having this problem and am quite sad to see it's been unresolved so long. :( RIP, save file. And condolences to y'all as well. I'll try to give as much background info as I can recall without having access to the file... If there's any other data that could be helpful, I'm happy to provide.
Platform: Switch. System software up to date as of this post -- v 10.2.0
Playing as Walani. The game crashed ("Software closed because of an error") as I attempted to enter the volcano. I have tried several times to re-open the save file, but the game crashes each time after a few seconds of the "Loading..." screen. My save file says Volcano 1-55, but like truemiang, I'm wondering if the date rolled back -- the dry season had begun. Sepia toned world, monsoon floods were drying up and I started to overheat on my way to the volcano. I had not gotten any eruption warnings yet.
I had entered the volcano previously in monsoon season and prepared my dry season station by the altar (endo pit, ice box, ice maker, empty bird cage, chest with some weapons & fuel. I think I left some jerky and coconuts in the icebox). I tried to re-enter a few days later with an armored boat and iron wind. Pretty sure I had the straw hat, backpack, and umbrella equipped, probably with some wetness from running into waves. No other status effects/buffs.
I'll try to remember what was in my inventory... definitely a thermal stone, a toucan, a black feather, a morsel, a few ice cubes, cut grass, logs, nitre, stone, flint, and a couple of Cutlass Supremes. I probably had my horned helmet, some yellow mosquito sacs and a few manure. Maaaybe a boat repair kit, dead rainbow jellyfish, a couple small and regular jerkies (is that a word?), and/or a few nearly rotted monster meats -- don't remember if these were used / left behind.
I sure hope there's a fix to this eventually! This game is brutalizing enough without bugs wiping out files that were going well, ha. Thanks devs -- I appreciate your time!