Have you tried purging your incomplete files, & also turning performance options off. If too many incomplete files can affect connection (UPnP fails.) (UPnp communicates with the modem/router to open a port for LW, but if too many incomplete files attempting to connect it can flood the communication with the router & UPnP fails.)

How many accounts do you have on your computer? If you have more than one, try using LW on one of those, else set up a test account. If LW connects, then copy 'that' gnutella.net file across to the other account. (I'd have suggested preferences folder, however the pointing of LW to a location the other account cannot access might cause permission issues.)