OK, so the story is that Oxwall is installed in /home/me/bev/oxwall, because I am trying to bolt it on to an existing web site - root /home/me/bev. I have bev in my hosts file as 127.0.0.1. However, the FB basic app configuration won't allow a site URL of
http://bev/oxwall, so I made it
http://localhost/oxwall, which is acceptable.
To check out the app I installed the FB php-sdk, and navigated to
http://bev/example.php. Then I got an intelligible error message saying that my app URL did not match the request.
So I changed my site URL to
http://localhost/oxwall, and then it worked, except that when after authorization it redirected back to oxwall I got a message like 'the email address is already used'.
Does that ring a bell?