Tvheadend

Have Kodi 17 on Windows, accessing TVH 4.2 on a RPi, with HDHomerun tuners.
Intermittently, I can't schedule a recording from Kodi. It fails with (from the log file):

17:41:23.883 T:8852 ERROR: AddOnLog: Tvheadend HTSP Client: pvr.hts - Command addDvrEntry failed: User does not have access
17:41:23.883 T:8852 ERROR: PVR - PVR::CPVRClient::AddTimer - addon 'Tvheadend:ariel:9982' returned an error: server error
17:41:23.883 T:8852 ERROR: PVR - PVR::CPVRClients::AddTimer - cannot add timer to client '1924689120': server error

Strangely, this only happens on some channels - at exactly the same time I can schedule a recording from another channel. The error seems specific to the channel. There are 4 tuners in all, so there are plenty of them.
What gives here?