Well I don't know what's going on , maybe someone else can help you. I would just stay on r478 for now until there is a way to get newer version to work. I'm on PS3ultimate CFW (3.55 Kmeaw with some extra features).
Ok, well it's not only me apparently that has this problem others have the exact same problem.
To quote: "I tried to load my FBAnext up and it freezes on a black screen the whole ps3 freezes not even the txt will show of cp1, cp2, neogeo etc.. I still have the roms for it so I'm not worried about deleting it and starting fresh but when I did the samething? Should I just delete the fbznext from my ps3 menu or connect with ftp and delete it all, Any help would be greatful thank you."
A possible fix: "The blank screen happens because it cannot find the rom path, you need to go onto multiman, go to /dev_hdd0/game/FBAN00000/USRDIR/
and copy the xml or (as of 2012) the cfg file. And edit the rom paths. Once the paths are correct, you won't have any more problems."
Thanks for trying to help. Hmm wonder what extra features your firmware has... might it just be a Kmeaw with mfw builder on top...