"The URL-encoded form data is not valid" error message

"The URL-encoded form data is not valid" error message

If you get this error it's because...

You (or some else) installed update MS11-100 (http://technet.microsoft.com/en-us/security/bulletin/ms11-100)
You have too many fields in your form. Way too many. More than 1000, actually.

The only real solution is to change your form so you don't have that many fields.

The temporary fix -- until you publish the eSpace -- is to add the line
<add key="aspnet:MaxHttpCollectionKeys" value="some number here"/>
in the <appSettings> in web.config.

For more info, Scott Guthrie to the rescue: http://weblogs.asp.net/scottgu/archive/2011/12/28/asp-net-security-update-shipping-thursday-dec-29th.aspx

Hope this helps!
I started to get this error now, and my form has 5 fields
I got that error too, and I doubt that I had anywhere near 1,000 fields in my form as well, but following the fix that Carlos recommends did fix the issue for me a few months ago.