Fixes the major "Invalid" error, now most saves should work, except those with time stamp issues...
Let's get started.
COPY YOUR ORIGINAL SAVEDATA FROM THE PS3 -
1. Convert your game with PS2iso2pkg; REMEMBER THE GAME ID. That's how you find it on your storage media (PS2UXXXXX).
2. Under "Game", navigate to "Saved Data Utility (PS2)," and delete the savedata for your PS2 game.
3. Load your converted PS2Classic at least once on your PS3, then quit when you see the PlayStation 2 logo.
DO NOT SAVE ANY DATA TO THE "MEMORY STICK" WHILE IN-GAME, THIS IS CRUCIAL.