My GameCube has several times told me that my Metroid Prime save files was corrupt.
I restarted my GC without the game, went into the memory card manager and tried to delete the file. It did not work, so I started the game again and the file was okay again. Hmm
I dont know if it works on PS2.. Try starting the PS2 without the game, go into the memory card manager, click on the save (do not delete though), go back, start the game and see if it works.
Its a loooong shot, but you never know.
Btw, is it a 3rd party memory card or a official Sony one?
Remember to ALWAYS buy 1st party stuff (SONY), especially when it comes to Memorycards.