Make sure both LW's are not using the same connection or listening ports. Check via Tools -> Options -> Advanced -> super really advanced > Firewalls. If both LW's are using either same listening or connection port, the clash will stop one from connecting.

Other than that, try deleting the gnutella.net file; If LW Will Not Connect (click on the blue link). It seems for LW 5 it's best if you simply delete that file rather than replace it. LW will create a new one. OH .. if you're back to using 4.18 then follow those directions & replacing the gnutella.net file. And firewall setting is Tools -> Options -> Advanced -> Firewalls.