HTTP Errors with .Net 3.5 SP1

I have a ASP.Net website that is failing on AJAX postbacks (both with ASP.Net AJAX and a 3rd part control) in IE. FireFox works fine. If I install the website on another machine without .Net 3.5 SP1, it works as expected.

When it fails, Fiddler shows that I'm getting a 405 "Method Not Allowed". The form seems to be posting to pages other than page I'm viewing.

The form's action is "#" for the page on the broken website (with SP1). The form's action is "Default.aspx" for the same page on a website without SP1.

Any ideas?

From stackoverflow Josh Clark

• Check out the following Microsoft Knowledge base article:

  http://support.microsoft.com/kb/216493

  If you're using IIS4 or IIS5 this may be the problem.

  From Bork Blatt

• SP1 changes the HtmlForm control so that it honors the action attribute, where previous versions ignored it. It sounds like you have something like this on the broken pages:

  <form runat="server" action="#">
  

  Remove the action, and it should be fine:

  <form runat="server">
  

  More info here: http://forums.asp.net/t/1305800.aspx

  From Dave Ward