I'm trying to get MythTV working in Kubuntu. Unfortunately I've got the following problem:

If I fire up the MythTV Frontend and select "Watch TV" a "Please wait..." label appears and after a while the screen falls back to the home screen.

tail -f /var/log/mythtv/mythfrontend.log prints out the following:

2010-10-14 19:22:18.809 MythContext: Connecting to backend server: 127.0.0.1:6543     (try 1 of 1)
2010-10-14 19:22:18.811 Using protocol version 23056
2010-10-14 19:22:22.641 TV: Attempting to change from None to WatchingLiveTV
2010-10-14 19:22:22.641 MythContext: Connecting to backend server: 127.0.0.1:6543 (try 1 of 1)
2010-10-14 19:22:22.642 Using protocol version 23056
2010-10-14 19:22:22.715 Spawning LiveTV Recorder -- begin
2010-10-14 19:22:26.563 Spawning LiveTV Recorder -- end
2010-10-14 19:22:26.565 ProgramInfo(): Updated pathname '':'' -> '1005_20101014192226.mpg'
2010-10-14 19:22:26.569 We have a playbackURL(/var/lib/mythtv/livetv/1005_20101014192226.mpg) & cardtype(MPEG)
2010-10-14 19:22:33.070 RingBuf(/var/lib/mythtv/livetv/1005_20101014192226.mpg): Invalid file (fd -1) when opening '/var/lib/mythtv/livetv/1005_20101014192226.mpg'.
2010-10-14 19:22:33.072 We have a RingBuffer

Then there is a whole bunch of those...

2010-10-14 19:22:33.186 RingBuf(/var/lib/mythtv/livetv/1005_20101014192226.mpg) error: Invalid file descriptor in 'safe_read()'

... before it falls back to the main menu.

I've got a MSI TV@Anywhere Plus Tuner Card (Philips Semiconductors SAA7131/SAA7133/SAA7135 Video Broadcast Decoder).

Any idea what could be the problem?