The logs appear to indicate that as far as the PMS side of things is concerned, things are set correctly. It detected your external WAN IP as 174.108.xxx.xxx and that you claim to have port 32400 available and forwarded to internal IP 192.168.1.100:32400. Up to that point, it all looks good. However, the logs then show the error:
Mar 11, 2012 12:53:02  DEBUG - MyPlex: Published Mapping State <--- 422
Mar 11, 2012 12:53:02  WARN - MyPlex: Invalid response when mapping state (code=422):<?xml version="1.0" encoding="UTF-8"?>
<Server name="Home-PC" host="" port="32400" machineIdentifier="xxxxxxxxxxxxxxxxxxxxx" version="0.9.5.3" />
<?xml version="1.0" encoding="UTF-8"?>
<error>Address timeout while connecting to 174.108.xxx.xxx:32400</error>
This indicates that the myPlex server was unable to connect back to your machine. When I attempt to see if I can get a manual connection to your IP:port via either HTTP or telnet, I'm likewise unsuccessful, which makes me think that the port forwarding isn't actually working correctly.
Now, just to verify, in your router settings, precisely what are your port forwarding settings?
Since your router supports uPnP, have you tried simply removing your manual port forward and letting the Plex Media Server auto-configure the connection? If so, what results do you get on the PMS preferences myPlex tab? Also, what are the "Plex Media Server.log" results?