Plex Cannot Find Media

Server Version#: DSM 6.2.2-24922 Update 2 - DS918+
Player Version#: PlexMediaServer-1.18.0.1944-f2cae8d6b-x86_64

After a recent update to the newest Plex version, Plex cannot find my media.

Launching Plex from the DSM menu lands me at the login screen (normal). After logging in, I cannot see any of my media. The screen blinks for a moment (every 30s or so) and there is a spinning icon, then nothing populates on the screen.