YEP. that’s one of those that happens EVERY time you try to access the server before internal initialization is complete.
It’s the Plex/web client being overly demanding and not recognizing the server is still in its Initial startup state.
Once the DB is ready and running, Plex/web will magically succeed
EDIT: Forgot to also mention that this is extremely common on slow NAS boxes (insufficient CPU to get things done quick enough)