If you see the above error when you’re trying to set up your DVB tuner card one cause is that MythTV can’t get access to the tuner card. This can be because there’s another application installed which has access to it, e.g. something like TVHeadend. On XBMCBuntu you can uninstall TVHeadend by running the following:
apt-get remove tvheadend
apt-get purge tvheadend
…then retry the setup. If that was the problem it should pick the tuner up now without issue, showing the tuner name in place of ERROR_OPEN.