The NAS is running DSM 7.x, correct?
Do you still have Plex running on your desktop or at least a copy of the Plex data folder from that system?
See FAQ 26. Read the entire procedure first. It clones the desktop PMS on the NAS (and wipes out any existing PMS installation on the NAS).
You will need to uninstall Plex from the DS920+ first, including the existing PlexMediaServer folder. Choose the “Erase” option when uninstalling.