Optimizations are starting to fail

I am having problems with optimizing specific titles. I’ve tried recreating the MKV files with and without various subtitles, but the optimization still fails. I’ve attached what I hope is a clean log file. This is a fairly recent issue, though the last two updates I’ve installed did not resolve it.

Plex Version 1.13.2.5154
Runs on a Synology DS415+
The application and MKV files are stored on the NAS
The optimization runs for a while, but eventually fails. When it fails, my RAM usage spikes though my CPU usage is low. The conversion process generally hits the processor hard, but I try to convert overnight.

Anyone know what could be wrong? As mentioned, most movies work well but a few lately have failed and continue to fail even if I make a new MKV file.

Thank you in advance!