Its most likely because you've modified that file on the disc. The PS2 is not like the PC, where you can go editing files without too much problem. Generally PS2 games require custom made tools to extract/rebuild certain files from larger compacted files on the disc, then to make sure the file's LBA is rebuilt, so the game still knows its a legitimate file. PS2 uses a lot of LBA checks that you need to make sure are still intact.
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