I have not posted a new blog entry in the last couple months due to the holidays and doing a lot of work travel in January.  This will be a short one, but I have seen a lot of customers having issues with this. A recent security patch, which you may already have installed if you are using automatic updates, seems to be having a bit of an affect on a number of Reporting Services reports.  If you have SSRS reports with a large number of parameters, you may be affected.  Your report will not render and you will see the following in the report server error log:  “System.InvalidOperationException: Operation is not valid due to the current state of the object”

This is due to the fact that the patch modified the default value of aspnet:MaxHttpCollectionKeys to 1000

There is no need to uninstall the patch!  You can simply resolve the issue by modifying the web.config for Report Manager (typically found in  Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportManager). Then simply modify the value to a higher number. 

For example:

<add key="aspnet:MaxHttpCollectionKeys" value="1000" />

modified to

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

This fix is documented in KB2638420

If you want to know the exact number that you require for MaxHttpCollectionKeys, a fellow MSDN blogger has documented an approach in http://blogs.msdn.com/b/paulking/archive/2012/01/16/using-an-http-module-to-assist-in-adjusting-the-value-of-aspnet-maxhttpcollectionkeys-imposed-by-ms11-100.aspx .

Hope this helps if anyone is scratching their head over reports that have run fine for years that are no longer working all of a sudden.