Problem with login

I'm running into a strange situation with AuthUser and logins.

I've used AuthUser in several wikis without issue.  I added AuthUser to 
a new wiki and it seemed to work.  I created a user and user group and 
updated config.php to specify that the group would have edit authority. 
Everything seemed to work OK.  In the browser window where I entered my 
login credentials, I can edit a page.  When I try to edit a different 
page I'm able to without having to re-enter credentials.  I presume that 
the previous login is recorded in a cookie in the browser.

When I attempt to edit pages from a different system or from a 
private/incognito browser window, I am unable to.  I am prompted for 
name and password.  When I enter them, the website takes a long time to 
respond, and then redirects to a bogus page.

The link for to the page displaying the login box is
   https://example.com//Main/HomePage?action=edit
After entering the credentials, and a long wait, the redirect is to
   https://main/HomePage?action=edit

I changed config.php to set fixed 'admin' and 'edit' passwords 
(essentially ignoring AuthUser), but I see the same behavior.

-- 
Michael Eager