Had same error on OSX.
Resolved removing manually the old instance of plex media server in Library/Application Support/Plex and the helper too.
It seems that some instances of the old versions remains and uses the same config files and/or ports.
Just removed the app, restarted and putted the new media server in startup items.
Now it works nicely.
Hope it helps.
This solved it for me too. I removed the old plex media server, the plex helper, and the two plex helper config files. I also made sure the new plex media server is in the Applications folder, rather than replacing the old one in Application Support. No system restart was needed either. Just make sure to close plex and the media manager, and quit plexhelper from Activity Monitor before deleting/moving files.
Hope this helps!
Edit: I should probably mention that I manually set the port to 32400 which is forwarded by my router, in case that makes a difference.