Reboot the computer.

Try deleting the complete preferences folder whilst LW is CLOSED. But before you do, make sure you make a note of important settings such as your saving location, days to keep incomplete files, speed settings, etc. Close LW. Do steps (a) & (b) below, then to the following -> Next time you open LW you will need to redo all your settings. During that process, set LW to not connect on start up. So then you are opening the program but not connecting immediately. Let the program settle for 5-10 mins. Then go to File menu, choose connect. Give it plenty of time to connect.

Before you choose connect. (a) Whilst LW is CLOSED! Also re-add the gnutella.net file as you did. Also add this one: Hostiles List

(b) Check the Windows system tray (usually located in the bottom-right corner of the screen, next to the time), and locate the icon that is associated with the security program that you are using. Right-click the icon to enter the settings. User Cant Connect - LimeWire

Edit: Your router may need rebooting or resetting. Depends on model how you do that. Perhaps turn it off for 10 mins then turn it back on.

It sounds like your router's UPnP is not working properly. UPnP is a communication process between the router & the program that allows the program to request the router which port(s) it needs to be opened whilst the program is open.