Hi metuckness,
I know your post is from 2 years ago, but I do have a similar problem with the webplugin.exe:
As long as I am in the same subnet everything works fine - as soon as I try to access my camara from another net (e.g. via ssh tunnel), I get the login page displayed, do the login, but it says "Login failed". Did you find a solution to your problem?
Cheers,
Selters