Ah, I'm not 100% sure of the details, but if you've
not modified the fonts, then I guess it could well be issues with character that the default font's don't support.
Having said that, my understanding is that 'Russification' of the 500 was a common operation, so with luck, you'll find some information to help either in the 500-Dev forum
here or failing that,
here on the Russian the-ebook.org Sony Reader forum.