I spoke too soon. I can access the RoundCube page at /webmail from within the network or a VPN connection using the machine’s private LAN address. I am port forwarding 443 to the server for the public IP address, and I am able to telnet to the server on port 443 from the Internet, but can not complete a connection with either Firefox or Opera to https://domain.com/webmail and have nothing in the debug folder to show for it. I do, on the first attempt after an install (I did both an upgrade and a clean install) get a warning that the site is using a self-signed certificate, and I have to approve it, but after that, the browser eventually times out.
Let me know if you’d like me to do anything. This exact setup worked with Yosemite and your previous version.