We recently upgraded one of our custom modules to work with DNN7.2 from DNN5.6.2. The file upload in the module is now broken, with this error: 

A critical error has occurred. Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> 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.

The html file upload control is rendered through AJAX on the page, and a linkbutton posts it to the server.

We are unable to determine if this is DNN related or not. The only changes to the module between the two versions are the .Net version (3.5 & 4.5) and use of newer file/folder management DNN libraries (FolderController vs. FolderManager). Is there anything in DNN that needs to be configured differently to get past this?
We set EnableEventValidation="false" in Default.aspx and the error did go away, but obviously that is the least desirable solution. We have also tried other suggestions for this problem in Asp.Net, like linkbutton, updatepanel, checking for IsPostBack in page load before modifying contents of a control, etc. It confuses us more that the same code was working with Asp.Net 3.5 and DNN5.6.2, but does not with Asp.Net 4.5 and DNN7.2. So it is less likely to be an Asp.Net issue, IMHO.

Thanks in advance for any inputs!