[mythtv-users] Error: Channel group '' doesn't have any channels defined.

[UB40D]

I have recently started getting the following message when trying to
display the upcoming programs from "Manage recordings / Program
guide":

  Channel group '' doesn't have any channels defined. The program
  guide will have nothing to show you.

One thing I did recently was a full rescan. Could this have caused
this nameless and empty channel group?

What's a channel group, and why do I need one? I never had to worry
about the existence of channel groups before. How do I create a
suitable channel group and populate it, if that's the way to cure this
problem?

Thanks in advance