I updated the guide by downloading the latest version of Plex (Synology media server), deleted all old scheduled recordings, re-added recordings.
Now only some recordings work. Others fail partway through the show and no recording is created. I noticed that an exclamation point appears in the Plex server where you would ordinarily see the recording progress. I can press the exclamation point and re-start recording, but then it fails again. I’ve been using the DVR feature since it was in beta and never had this problem.
I’ll post a log next time this happens–debug was disabled…
Server Version#: 1.21.1.3876
Player Version#:  N/A
Tuner Make/Model: HDHomeRun CONNECT QUATRO
Guide/Lineup name: ?
Using XMLTV?:  ?
Channel number/Name:  Various