SSRS 2005 running on Windows Server 2008 R2 had the following errors as reported on SSRS Logs:-
System.InvalidOperationException: Operation is not valid due to the current state of the object.
Seems to possible causes:-
Installation of Security Update for Microsoft .Net Framework 4 on Servers (KB2656351) and also having large number of parameters or form fields in a SSRS report.
Fix
Open Web.Config for ReportManager usual at:-
C:\Program Files\Microsoft SQL Server\MSSQL.2\Reporting Services\ReportManager
Find <appsettings> in the file and add 2 new keys:-
<add key="aspnet:MaxHttpCollectionKeys" value="10000" />
<add key="aspnet:MaxJsonDeserializerMembers" value="10000" />
So you should see something like:-
<appSettings>
<add key="ReportViewerServerConnection" value="Microsoft.ReportingServices.UI.WebControlConnection, ReportingServicesWebUserInterface, Version=9.0.242.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />
<add key="ReportViewerTemporaryStorage" value="Microsoft.ReportingServices.UI.ReportViewerTemporaryStorage, ReportingServicesWebUserInterface, Version=9.0.242.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />
<add key="aspnet:MaxHttpCollectionKeys" value="10000" />
<add key="aspnet:MaxJsonDeserializerMembers" value="10000" />
</appSettings>
Also I recommend changing the Web.Config for ReportServer usual at:-
C:\Program Files\Microsoft SQL Server\MSSQL.2\Reporting Services\ReportServer
In this file there is usual no <appsetting> already added so just pop one under the </ConfigSections> part:-
</configSections>
<appSettings>
<add key="aspnet:MaxHttpCollectionKeys" value="10000" />
<add key="aspnet:MaxJsonDeserializerMembers" value="10000" />
</appSettings>
No need to re-start SSRS or IIS, it should just now be fixed.