Recently my Plex server has started to town an error anytime you try to convert something that “Not enough CPU for conversion of this item” - although the item isn’t anything heavy at all. For example:
Users and even myself and reproduce this. I have an i7-3370k which isn’t the most powerful but shouldn’t have any issues converting a 1080p video on the fly.
If you updated DSM (automatically or manually), Synology often resets certain permissions, including but not limited to UDEV which I use for GPU/ASIC access.
Reinstalling the app reasserts the needed privileges.