I just also ran into this issue as well (have had the nvidia plugin driver installed and working fine, plex working, etc). Came to find Plex stopped and refused to start. Even tried a remove and reinstall and discovered nvidia driver was the issue. In the plugin I was showing the driver installed and on the latest version. However it also stated nvidia-smi failed error. Further research shows that the latest driver has dropped support for several older cards including my Quadro P2000 it seems. After changing to the production driver v580.119.02, allowing it to download/install and a reboot, it now shows my Quadro P2000 and Plex is running once again.