Recording fails with "There was a transcoder error"

I just set up a new Plex server in a NAS4Free jail, and I decided to try the DVR functionality. I started testing by recording programs already underway in the Program Guide. When I do that, the Recording Schedule page shows that the program is recording. After a minute or two, the Recording Schedule displays a yellow triangle and exclamation mark over the recording and shows the error message “There was a transcoder error.” The recording never appears in the library.

Following is what seems to be the relevant data from the log files after attempting to record “Thomas and Friends”. Sorry, that’s what was on the air at the time.

Sep 06, 2017 13:38:34.978 [0x80b0fc800] DEBUG - buildLiveM3U8: min -1 max -1
Sep 06, 2017 13:38:34.980 [0x80c24d000] DEBUG - buildLiveM3U8: min -1 max -1
Sep 06, 2017 13:38:34.981 [0x80b0fc800] ERROR - [Transcoder] http://127.0.0.1:32400/livetv/sessions/f12ffa46-c283-4cec-9225-c70cbf116df7/aeafd513-d5a3-41c4-abc7-355a3def1e3b/index.m3u8?X-Plex-Token=xxxxxxxxxxxxxxxxxxxx: End of file
Sep 06, 2017 13:38:34.982 [0x80a7d3b00] ERROR - JobManager: child process returned: 1 (Operation not permitted)
Sep 06, 2017 13:38:34.982 [0x80a7d3b00] DEBUG - Jobs: '/usr/local/share/plexmediaserver/Plex Transcoder' exit code for process 67862 is 1
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - Jobs: '/usr/local/share/plexmediaserver/Plex Transcoder' exit code for process 67862 is 1
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - DVR:Recorder: Asked to stop recording 'Thomas & Friends - Kindness With Friends'.
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - DVR:Recorder: Stopping transcode session.
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - Job was already killed, not killing again.
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - Job was already killed, not killing again.
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - DVR:Grabber: Freed a tuner (now 2 available)
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - DVR:Grabber: Recording for 'Thomas & Friends - Kindness With Friends' stopped with status error.
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - DVR:Grabber: Using recording status.
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - Activity: updated activity 31212bcc-349d-4a55-845e-dd7ada0397ec - completed 100% - Recording
Sep 06, 2017 13:38:34.982 [0x80c44cf00] DEBUG - DVR:Grabber: Scheduling 'Thomas & Friends - Kindness With Friends' for assimilation.
Sep 06, 2017 13:38:34.986 [0x80b24f400] DEBUG - DVR:Subscription: Failed to resolve subscription for 'grandparentGuid: com.gracenote.onconnect://show/186061 grandparentThumb: http://tmsimg.plex.tv/assets/p10617722_b1t_v5_aa.jpg grandparentTitle: Thomas & Friends grandparentYear: 1984 guid: com.gracenote.onconnect://episode/EP005236340551 originallyAvailableAt: 2011-04-09 parentIndex: 2011 title: Kindness With Friends type: 4 year: 2011' to library item
Sep 06, 2017 13:38:34.986 [0x80b24f400] DEBUG - DVR:Grabber: Postprocessing, we're going to put `Thomas & Friends - Kindness With Friends` in `/mnt/media/dvr/Thomas & Friends (1984)/Season 2011/Thomas & Friends (1984) - 2011-04-09 00 00 00 - Kindness With Friends.ts`
Sep 06, 2017 13:38:34.987 [0x80b24f400] DEBUG - DVR:Grabber: Operation for Thomas & Friends - Kindness With Friends completed with status error (There was a transcoder error)
Sep 06, 2017 13:38:34.987 [0x80b24f400] DEBUG - Activity: Ended activity 31212bcc-349d-4a55-845e-dd7ada0397ec.

The tuner is an HDHomeRun Dual (HDHR3-US). I have an identical spare, so I configured Plex to use that instead. I saw the same error.

Here’s my configuration:

Plex Media Server version: 1.8.1.4139 in a jail
Host system: NAS4Free 11.0.0.4 running on first gen Mac Pro
RAM: 18GB
Disks: 4x 2TB in RAIDz2 with 6.53TB free

I made the “plex” user the owner of the library directory where the DVR recordings should be saved.

Is there anything that stands out as being wrong?