Operation is not valid due to the current state of the object.

[InvalidOperationException: Operation is not valid due to the current state of the object.]
   System.Web.HttpValueCollection.ThrowIfMaxHttpCollectionKeysExceeded() +2725978
   System.Web.HttpValueCollection.FillFromEncodedBytes(Byte[] bytes, Encoding encoding) +61
   System.Web.HttpRequest.FillInFormCollection() +148

[HttpException (0x80004005): The URL-encoded form data is not valid.]
   System.Web.HttpRequest.FillInFormCollection() +206
   System.Web.HttpRequest.get_Form() +68
   System.Web.HttpRequest.get_HasForm() +8807863
   System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull) +97
   System.Web.UI.Page.DeterminePostBackMode() +63
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +133

This error is due to a recent Microsoft Security updated that limits how many form fields can be included in an Asp.Net postback.  If you are using the PDshop Admin and bulk updating a category's items, you may encounter this error if there are many items assigned to the category.

To workaround this new Microsoft limitation, add the following key to the appSettings section of your web.config, and increase the value as needed:

<add key="aspnet:MaxHttpCollectionKeys" value="10000" />