following issue is encountered .

JavaScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: Operation is not
valid due to the current state of the object.

fix : Add below appsettings in web.config

<appSettings>
 <add key="aspnet:MaxHttpCollectionKeys" value="2000" /">
<add key="aspnet:MaxJsonDeserializerMembers" value="2000" /">
</appSettings>

reference:

http://codecorner.galanter.net/2012/06/04/solution-for-operation-is-not-valid-due-to-the-current-state-of-the-object-error/