Sometimes deleting the preferences folder is the only fix needed... if you're sure that you deleted the folder properly (if all LW's settings don't revert to default on restart, then the folder hasn't been deleted properly) then try changing listening port... on the same page where you checked whether UPnP was enabled or Manual Port Forward

In LW tools > options > advanced > super really advanced > firewall. Change LW's listening port to 65000 (or something in the range of 49152 - 65535) > ok.

Also log into your router & double check that UPnP is still enabled within your router's configuration.

If that doesn't help, try port forwarding. Since you can't connect with other p2p apps, maybe the problem lies with your router.