In ASP.NET 3.5 you may encounter one of the following error messages:
“Operation is not valid due to the current state of the object”
“The URL-encoded form data is not valid”
Microsoft released a Security update at dec 29 2012. The update contained (amongst other features) an upper bound to the number of simultaneous HTTP form elements that can be posted. The new limit is 1000.
In the Sitecore Page Editor you may experience that for large pages you have more than 1000 form elements.
WHAT TO DO?
To fix this you should add the following to the web.config:
In the <appSettings> of the web.config add the following:
<add key=”aspnet:MaxHttpCollectionKeys” value=”value greater than 1000” />
Thanks to Alin Parjolea for the tip.