moreira, firstly I will never, ever know as much about this stuff as TLS dude, but I'm just curious to shed a bit of light...
So you mean right back surround only? Left surround and left rear is working?
Is this happening on multi channel 5.1/7.1 tracks from DVD's? Or is this happening when the receiver is trying to matrix 2-ch into 5.1/7.1? Or both?
Have you tried re-setting the receiver?