Yeah i got it quite confirmed that this might be the problem.
Not only SCUMMVM was affected by this, other homebrews like roms for emulators and such that didn't run earlier got working once i freed space, moved the files from the PSP drive and back again so it was rewriten on the flash card.

You don't happen to have a 8gb memory card? I too had this problem with SCUMMVM just turning the PSP off when executed so i made some research on the net and found out that the PSP can have problems reading homebrew that is past the 4gb on the memory card... Don't ask me why since i'm not an expert,...