Operation is Not Valid Due to the Current State of the Object

Resolution

If ever you get an exception of ‘operation is not valid due to the current state of the object’ on a XMPro form, or any asp.net form with a huge number of controls, you need to set/add in the web.config file:

<appSettings>
<add key="aspnet:MaxHttpCollectionKeys" value="1001" />
</appSettings>

Increase the value until the problem goes away.

This is a security update from Microsoft link here.