That looks like the WebOS version, not the Plex app version.
Authorized Devices is at https://app.plex.tv/desktop#!/settings/devices/other.
You should see something like:
The current version of the LG Plex app is 5.6.1, released on Nov 24. You should have it, since you removed & re-installed, but always nice to confirm.
One other thing to try is turning off the TV, then removing power for ~30 seconds. That completely clears running/background apps from the TV memory. Somewhat like shutting down a PC versus putting it to sleep. Try it once. It will either help or not. I’ve had my LG for 3 years. I’ve had to pull the plug maybe twice. You mentioned rebooting PC in your OP. Not sure if that meant removing from AC power.
