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

Here is a suggested solution I found on another post. It works well on mine.

You don't have to uninstall the patch; the default value of MaxHttpCollectionKeys is set to 1000 by the security patch. You can override the value by adding the following key to the appsetttings section in your SSRS report manager web.config file (usually in Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportManager)

<add key="aspnet:MaxHttpCollectionKeys" value="10000" />

10000 is just a suggestion, you can set it to the value of your liking.

We did this change and the reports worked for us.

Regards,

Sri