virtualmin "nano" error opening terminal unknown

The popup terminal is not an interactive terminal (yet). It can only be used for non-interactive tasks. All editors are interactive (well, ed or sed isn't, but that's not what folks have in mind when they think "editor"), and thus can't be used in the popup term. I'm surprised vim would even try to start...I would think it would complain about the terminal and exit.

The popup terminal is just a nicer frontend to the Command Shell module. The is a terminal you can run within Webmin that allows interactive use, called AJAX Term, which spins up a special server in the background to interact with the terminal. And, of course, you can use the editor in the File Manager to edit files in a pretty nice way...it's not vim, but it has good syntax highlighting and handles the basics for quick and dirty editing.

I know it gives the illusion of being a full terminal, but we just don't have the backend capabilities to handle interactive use yet. It's vaguely in the plans to add it, but it's a ways off. We either need to spin up a dedicated Web Sockets capable server for every session (the way AJAX Term does), or we need to add support to Web Sockets in Webmin's miniserv web server. Neither is trivial. The fronted also needs quite a bit fo work to become a full-featured terminal, though we may be able to borrow existing open source code for the VT102 emulation.