I noticed that on every failure I see something in the log that says something like this:
ERROR: PVR - AddTimer - addon '172.16.1.3:connected' returned an error: the command failed
NextPVR is definitely running (I can connect to it using XNEWA and, as I said, a record once recording works).
BTW, if I paste the URL from one of the failed attempts into my browser, I get the following XML response from NPVR:
<err code="2" msg="Invalid Args"/>