I think i found a fix. Just a casual FM player, i dont work for Sega or anything.
In your game (even if it's the one that you tried to save) follow these steps exactly:
1.go to preferences (top right, click the "FM" button) and search for "cache"
2.uncheck "Use caching...(blah blah)" there should be only one option
3.Then save via "save game as" and save over the current save (this is important i think, i did a few tests and it only seems to work this way consistently)
this seemed to work for me.
I think the issue is just that caching flag, i dont think the actual save game is corrupt.
since you can save the same game if that flag is set to false (but i only got this to work consistently if you write over the same file via save game as)
I did a test by going on holiday for 7 days with that flag set to true and false. (about 4 times)
after each 7 day holiday i tried to save.
with it set to true it failed regularly, with false it worked consistently if did save game as and wrote over the existing save.
i only did about 10 tests tbh, so not enough to say that it is definitely the reason (not sure about my negative test per say since i cant be sure of the baseline, since im a user),
but it might point you in the direction of where to look.