I've read of others with a similar problem.

Does each user account use a different internal or external ip address? Check external ip address with a site such as What Is My IP Address? - IP Address Lookup, Internet Speed Test, IP Info, plus more from each user account. The reason I am asking is because I presume you are using a router or modem with NAT. Routers are ip based.

If a single user uses a specific program connecting through a particular port of the router, then the router reserves that port for that program & user (ip address.) If other users attempt to connect (or listen) thru the same port then they are blocked out.

I guess you could try using a different connection & listening port for each user. Not sure if that will work, especially since UPnP tends to find ports of its own. But you could try setting to UPnP in LW, and also choose another, under LW's tools > options > advanced > firewalls.