You may want to check the disc for any dirt or scratches- maybe that portion of the disc is obscured enough by something like this to access that area.
It's also possible that it could be an issue with the game save- you could try starting a new profile.
If none of these help then it's time to check the PS2. Flip open the top and look for any dust/ dirt on the lens that could make it harder to red the disc. I would use a DVD lens cleaning kit for something like this, available at most department/ electronics stores.