I remember I used to get the same thing (or similar), it says something about UPnP right?, make sure your server has UPnP enabled (it's enabled by default) AND make sure your router also has UPnP enabled (it's located in the LAN tab in your router settings), restart your router after that, if that doesn't work disable UPnP in your router and server, both methods will work, you won't get the port error anymore
Just to make sure, here's a check list:
- Portforwarded UDP 16261
- Portforwarded TCP 16262 - 162XX (it being the player capacity in your server)
- Portforwarded BOTH 8766 (this is not needed, but I did it just in case, it's the steams port I believe)
- Make sure UPnP is enabled
- Added all the ports to your firewall (to allow connection)
- Restart the router just in case
If that doesn't work, try hosting via the "Host" button, it works for me