URL-encoded form data is not valid
The MS KB issue in question is: http://weblogs.asp.net/scottgu/archive/2011/12/28/asp-net-security-update-shipping-thursday-dec-29th.aspx
In a nutshell there is now an upper bound on the number of simultaneous HTTP form elements that may be posted. The default is now 1000 without explicitly changing it with this key in the <appSettings> portion of the web.config:
<add key="aspnet:MaxHttpCollectionKeys" value="some number greater than 1000" />
There was a microsoft update and it may have caused your issue see link.http://knowledgebase.solarwinds.com/kb/questions/3476/Website+Error%3A+The+URL-encoded+form+data+is+not+valid