Finally I have found a way to create screencasts in Linux. Within Debian I didn't get it working at all. But under Ubuntu the "Desktop Recorder" (aka gtk-recordmydesktop) finally gave me a working OGG video file. Hooray. The first screencast I have recorded deals with the Zabbix monitoring software. Enjoy.
If you wonder: I have converted the OGV file into an FLV file for playing in the browser using ffmpeg on Ubuntu. Strange enough: the OGV file is 25 MB while the FLV video is 90 MB. I wonder if there is a way to play OGV files directly in the browser. But apparently that's not even decently specified in HTML5. Maybe I should use MP4/H.264? Meh, I suck at web video formats. :(