“We certainly test on ie 6,7,8,9, and would love to get your situation smoothed out.”
The scary part came a little while later…
“The cause is likely to be configuration based on the browser. Browser security settings should be set to default for testing. Temporarily disable all toolbars and add-ons. Clear caches and cookies (at least for your site domain and rpxnow.com.”
Oh yeah. I've heard that one before. So I was a bit skeptical.
On the other hand, I happened to be in a crowd and asked some people nearby with Windows 7 to see what happened to them when they tried to log in. It was one of those moments. Everything worked perfectly for everyone but me…
Gathering my courage, I pressed the dreaded configuration reset button as I had been told to do:
Then I re-enabled all my add-ons as janrain suggested. And… everything worked as advertised.
So there you go. Possibly I did something to my IE config at some point – I do a lot of experimenting. Conclusion: if any of you run into the same problem, please let me know. Until then, let's consider the incident closed.