As I was discussing with Tom in live support, I'm setting up Ad Sharing on a single site. All is okay on the admin side. When I log in as my test user -- contributor status -- and go to the advertising page, I paste in the ad code, hit the Save Changes button, and the window erases. Naturally, no ad shows up.
I don't think it's the code. Code from the save provider -- Amazon -- from two different accounts does work on the admin side but not on the user side.
And I don't think it's a browser issue. I've logged out, done it in different browsers and even on different computers.