Tompa,
Please look a new build of TVServer (SVN14650 or greater). It has enhanced debugging for DVB-T devices.
Also, please teste the folowing scenario if you can: Start Watching DVB-S, start recording on DVB-S, Change to DVB-T channel and go back to the channel being recorded (DVB-S). It's the inverse of the current scenario.
Please report if it fails too.
Sorry for the many round-trips, but I have no DVB-T device to debug it right now. If the issue also happens on DVB-S it might be easier to debug.