The reason for this error is the default value for MaxActiveReqForOneUser key in RSREPORTSERVER.config file is 20 which need to be changed to 0 (unlimited access) or increased to some higher value. After changing this setting IIS need to be reset in the reporting server machine.
Reportserver.config file exist in the following path (path may be little different based on the sql server reporting server version that is installed).
C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer
Reference:
http://www.sqlservercentral.com/Forums/Topic410741-162-1.aspx#bm507713