I was recently onsite visiting one of my customers when a rather interesting issue was brought to my attention. The customer was running Reporting Services 2008 R2 in SharePoint Integrated Mode and was testing a very simple report that contained a single parameter with 485 possible values. If they selected 484 of the possible 485 parameter values, then the report rendered exactly as expected. If they selected all 485 of the possible parameter values, then Reporting Services returned an error similar to the following:

Webpage error details

User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT
6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR
3.0.30729; Media Center PC 6.0; .NET4.0C; InfoPath.3; .NET4.0E; Tablet PC 2.0)

Timestamp: Tue, 15 May 2012 17:49:04 UTC

Message:
Sys.WebForms.PageRequestManagerParserErrorException: The message received from
the server could not be parsed. Common causes for this error are when the
response is modified by calls to Response.Write(), response filters,
HttpModules, or server trace is enabled.

Details: Error parsing near '

<!DOCTYPE html PUB'.

Line: 5

Char: 62099

Code: 0

After searching around quite a bit (with very little luck), one of my colleagues pointed me to KB article http://support.microsoft.com/kb/2661403. The error messages there didn't exactly match the error message that we were seeing, but it was close enough that we decided to test adding the following value to the <appSettings> section of the web.config file for the IIS Server:

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

After saving the web.config and running IISReset, we were able to successfully run the affected reports without encountering an error.