Sorry for bumping this old thread, but I was struggling with the white screen problem and (finally) found a fix which I didn't see mentioned anywhere else. Just in case anyone is still having the same issue, I thought I'd post here.
The file \_aio\fonts\tahoma.pcf changed between 1.4.1 and 1.5. I always get whitescreens and other weirdness when using the new file (463,468 bytes), but when I replaced it with tahoma.pcf from 1.4.1 (188,208 bytes) everything is fine. This is the only fix which works for me, and I've tried all the others like formatting with the Panasonic SDFormatter, updating the loaders, and enabling cheats and soft reset. Hopefully this will help someone else too.