This exact same issue also occurs for me (771 error on some channnels, but others work fine), but only very occasionally (once per month maybe). I'm fairly sure that it has something to do with a bad cable.
I have 2 receivers, and the one in the bedroom is not affected, so I know it has nothing to do with the dish or where it is pointed.
I have figured out how to fix it, for whatever reason, this works for me every time :
1. Unplug the receiver that is having the problem.
2. Unscrew the 2 coax cables that are connected to the switch that goes to the TV. (I think one says "To IRD" and one says "Out to TV" or something similar, but I'm at work).
3. Screw them back on.
4. Plug receiver back in.
5. Wait for reboot to complete, sometimes this takes 10+ minutes.
6. 771 error gone. Watch TV. Hope that something you tried to record didn't get screwed up.
I've been having the samething on my hr20-100. Looks like there was a software update last night now my tuner 1 is dead.