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

Whenever you see this error

 

add "aspnet:MaxHttpCollectionKeys" in web.config file or app.config file in your application as shown below

 

Solution:To solve this error, increase MaxHttpCollection value. Try adding the following setting in your web.config's <appsettings> block.

<appSettings>
<add key="aspnet:MaxHttpCollectionKeys" value="3000" />
</appSettings>

you can change the value accordingly as per your need.

Thanks,
Praveen

Reference

http://dotnetknowledgebox.blogspot.in/2012/10/operation-is-not-valid-due-to-current.html