After working fine for a couple of years I NOW get the following error message appearing when I select the Upload button in Admin>File Manager: 

A critical error has occurred.
Invalid postback or callback argument. Event validation is enabled using in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.
-----------------------

Can someone please advise how I can fix this so we can upload files via Fiel Manager again.

Thanks
MikeT