There is a problem with the Intel video driver and the J4125 CPU that interferes with hardware accelerated transcoding. Until Intel fixes their driver, the workaround is to fallback to the older driver.
See Synology FAQ 26.
You can also turn off hardware accelerated transcoding. You should be able to transcode lower bit rate H.264 1080p video using the CPU, ~10 Mbps or less.
In addition to the FAQ, the Installation & Setup support document also has good information about running Plex on Synology NAS:
Additional References:
DS720+ has J4125 CPU per Synology.
Celeron J4125 Passmark = 3238.
Passmark ~2000 is needed to transcode 10 Mbps 1080p video on CPU. Ref: What Kind of CPU do I need for my Server?
