I am not 100% of this but it might be correct. The times where I've had that error is when I've just searched for channels in MythTV-setup and it's receiving the EPG. This should mean that it's using the tuner to download all the channel data. I usually just wait and do something else for some hours and reboot before trying again.
If you ever get the problem of being in a vicious cycle when launching MythTV (where the display cycles between the "1" screen, "Watch Recordings" and I think another one) I think it's due to one of the cards that don't have a proper starting channel - or something along those lines that one card isn't properly configured. Solution here is to configure it or delete it from mythtv-setup.
Also, which card do you have? And, have you searched/found some channels? Just because a card is detected doesn't mean it's working properly unfortunately. I think in most cases it does, but not all. I have a HVR-4000 that got detected. However, DVB-S still required me me to change the firmware and DVB-T (even though found) cannot work since there's some kind of bug in the kernel.