Try formatting your SD card. Remember to back up everything first as formatting deletes ALL data on the SD card. This is just one idea, and not guaranteed to work, but try it.
Tried it, didn't help. I narrowed it down to a problem with the fighter folder, so I assume that there's a corrupt texture in there somewhere...though it could be something else, I'm not sure.
I'm currently editing a Brawl Iso to use with my USB loader. It seems to be stable so far, but I'm having a bit of trouble with replacing some files. Anyone know of a guide on how to do this?