FIX: Fix Plex Media Server Remote Access / Unable to Connect to Plex from The Internet (code=-52)

Monday, April 6, 2015
You've opened the ports on your firewall and verified that they are open via an open port check tool on the Internet.
You've tried Why am I locked out of Server after password reset or device token removal. You've successfully pulled out many locks of hair (optional).
You've even turned on UPnP on your router, just to test (you can turn it off, just make sure the port is opened and forwarded correctly).
Error 52 / code=-52
Your log has this line:
WARN - MyPlex: Invalid response when mapping state (code=-52)
What's happening
Some part of the Remote Access registration process fails if the host operating system is configured to use Jumbo Frames. If you don't know what that means, that's OK. I'm running an OpenSuSE Linux box as my Plex server and I am not a Linux expert, so I can't even really tell you where to go (if you know, leave a comment, please!).
For OpenSUSE, do the following:
$ sudo yast
... Your password ...
In yast, go to Network Devices and select Network Settings.
Select each network adapter and choose Edit.
Go to the General tab and change the MTU to a value that will prevent Jumbo Frames (1400 is a safe test value for most people, but you can google your optimal based on the kind of broadband you subscribe to).
Exit and try it all again (I restarted the plex server as well, just for good measure).