I deleted both the physical and virtual directories on that machine, then re-deployed it in a different path under a different virtual directory name (to avoid any kind of memory the system may or may not have); the error was the same.
I then tried deploying it on a different machine, using exactly the same process and the same web.config (except changing FullGeminiURL to match the new machine), and it worked. So it's obviously some configuration issue or application conflict on that first machine; which is unfortunate, because I'd much rather have it on that first machine than the second one.
The process I followed both times, in case you're interested, is this:
1) Create a new directory on the hard drive
2) Copy the contents of the <Gemini Install>\Deploy\Step2_WebApp to the new folder on the hard drive
3) Right click on the folder, properties, and turn on web sharing, with a new virtual directory name
4) Still in properties, make sure the ASP system account and anonymous internet user have read and execute permissions on the directory
5) In IIS, under the virtual directory's properties, make sure it's using the correct ASP version (both were without change), make sure anonymous access is allowed, and add Default.aspx as the default document for the directory.
6) Copy a web.config set up for how I want it configured into place
7) Make sure the FullGeminiURL property is correct (the second install required a change here, since the config was intended for the first machine)
At that point, the second machine functioned and allowed login, while the first gave the error I listed above, listing machine.config as the cause.
