I've noticed QUITE a few "corrupted save" issues on the internet. My save corrupted as well. I googled for a fix, and didn't find much but "you're stuffed, start again", or "here, have a save". I decided that someone should make a list of ways to fix it, so they don't have to search like I did, and where better then the Dissidia wiki? These probably work for other games too.
As I mentioned, my save had gotten corrupted. I couldn't find anything of use to me, so I had a look in my saves folder. I discovered an oddly named folder. It was something like "_OWRITE_00000000001384399863" I simply renamed it to its original name "ULES01270GameData00"(for PAL version, at least) the 00 at the end should be changed if your save wasn't in the first slot. For example, slot 3 would be 02. 1st slot is 00. IT WON'T WORK IF YOU TRY TO NAME IT TO ANOTHER SLOT, IT WILL STILL SAY IT'S CORRUPTED.
(Yes, I know this is a very unprofessional sounding article. Please make it better for me, I'm no good at these things.)
(K7TZ edit) I'll add my fix to this. In my case my save file works on ppsspp but corrupt on psp. all I did was manually save on ppsspp then transfer it to psp then it works fine.