SharePoint 2013 Webpart page:"Operation is not valid due to current state of the object'

Hi

 I tried to add an Custom webpart(Created using Visual studio 2012 with just 2 textbox controls) in SharePoint 2013 webpart page. But When i did that getting the error as

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

I added an below  entry in web.config file as suggested in few posts

<add key="aspnet:MaxHttpCollectionKeys" value="5001" />

But then also i couldnt able to add my web part to the page.

How to resolve this ?

Regards,

Siva