Here are my thoughts, copied and pasted from another forum.
It's not a decryption issue with the EBOOT for sure, but I think it's definitely a decryption issue. If you look at BBS1.DAT (BBS2.DAT and BBS3.DAT as well) in a hex editor, you'll notice they start of with a the header 00504744 (PGD). This is the same header for an encrypted ISO stored in a PSN download.
Now this is just me guessing, but the method of encryption for PSN downloads (which is doubtless the same encryption being used here) uses the act.dat in the flash to decrypt the needed content so that it's tied to the PSP. However, since this content hasn't been encrypted for a single PSP, the common decryption key must be somewhere. My guess is OPNSSMP.BIN has something to do with it, or maybe there's a new UMD encryption key that's stored on the 6.20 firmware. It could be possible that the three DAT files are dummies (though it's unlikely considering the game) and the crash is due to a new custom firmware check. I have no idea how to get a debugger running on the PSP, or if there even are any decent debuggers that work on commercial games, so I can't investigate much further.