Aborted JSON results on IE9

I had a strange issue last week when a jquery ajax call was being aborted only on IE9.
I did not have this issue when testing on Firefox, Chrome and Safari.

After some head banging, the solution was to switch from Cassini to IIS Express.
Serves me right though. What the heck was I still doing with Cassini?

I normally change to IIS Express or to IIS almost immediately, but I had forgotten in this case.