From the misbehaving computer, when I enter my username and password, then click the Login button, the screen changes to a temporary screen that tells me "You have been successfully logged in." In the upper-right corner of that screen it properly provides me with a button that reads "Logout [ username ]". Instead of then taking me to the forum index page, it returns to the login page. However, at the bottom of this new login page it indicates at I am now logged in as a registered user, yet the upper-right button simply reads "Login" (changed from "Logout [ username ]"). After three attempts at logging in, I'm told that I must now enter a confirmation code from a displayed graphic ... which doesn't actually display (there is a red-x graphic placeholder).
Why would one computer misbehave like this while the other two work just fine?
Login Screen
Success Screen
Revert to Login Screen
Confirmation code graphic non-display