There appears to be a bug with ExternalInterface when your Flash movie is inside a FORM tag, which is what ASP.NET does by default.
You will get a “null has no properties” type error because the JavaScript that ExternalInterface generates at runtime is unable to reference the Flash movie.
It appears to be due to IE not putting the form in the same place as other browsers’ DOMs (specifically the window object).
Either way, the workaround is to take it out of the form, or if not possible you can try several other things such as this SWFObject fix. Worst case scenario is to reference the Flash movie using document.forms[0].moviename.
There are many solutions and some discussion in the livedocs.
I have another ExternalInterface bug listed here in a previous post.