I too have multiple pvr150's hoked up to dish network boxes. The only time I see that error is when the pvr card is recording something on mythtv. Make sure your recording schedules do not conflict with the pvr150 "LiveTV" feed you are using. I have 4 receivers, well 2, and 2 RF second feeds. I have made a habit of scheduling my recordings on the pvr150's connected to the RF tuners, as I cannot control those with IR. I set the RF feeds to auto-tune to the channel I am recording and use MythTV scheduliing options to use specifically that tuner. It's a PITA to not have RF control, but if you have IR boxes, those are nicer to distribute as they can be controlled by the MD's as far as channel changing. Meanwhile the RF tuners do all of your recording.
Just a thought.