The firmware supplies a lot of the API calls to the hardware, in most cases the later games use these calls so running an old firmware wont work. The other side of the coin is that it forces users to upgrade to the latest version to play games and used as an anti priacy tactic.
I find it odd that you downgraded to 1.52 rather then 1.5 which is the firmware version that the downgrader was designed for.
Also note that all PSPs are region coded in terms of UMD films. If you have a JAP PSP and a USA UMD film, it wont play because USA is region 2 and JAP/UK is region 1.
You can try the Firmware version spoofer that is around to fool the game tha it is running on a higher version then it is.
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