Try to uninstall PMS from the Package Management Center in the Dashboard.
After that log into your server through SSH (4747) and
Select Plex and intall it.
The shell may be a bit stuck at "Updating system ...".
After a while, maybe the box screen will show up, close it and refresh your dashboard.
Plex icon should be green and the redirection should work too.
You maybe need to set up the SSH tunnel with the port 31400 (I can't test it at work).