posted 06-29-20 08:08 AM EDT (US)
Thought I'd post this here in case anyone else runs into the same issue and perhaps the same fix works. My attempts to host have, for a while, ended with the other player getting the "unable to join the game because there was no response from the host" error. I could join their games without issue.
Turns out, RoL might sometimes listen on the wrong network interface. After disabling my VM's virtual network adapters, everything works perfectly.
Both players are on Win10 May 2020 update on a LAN, and have Windows firewall running, with a rule allowing traffic in/out for the main RoL .exe. We haven't needed anything beyond install, patch up, add firewall rule, and install MSXML4.0SP3. (Except having to spam ok a few times on the firewall prompt in game.)