Tvheadend

On ubuntu 18.04, tvheadend 4.2.8-36~g5bdcfd8ac~bionic from the tvheadend apt repository. Many recordings work fine but sometimes I get "No input source" despite having two tuners and having nothing else recording or being watched. The logs don't seem to have any hints, what can I do to track down what causes this problem? Both USB WinTV HVR 950Qs which show up as Auvitek AU8522 on tvheadend.