Possible workaround:
In Reporting's web.config, make the following changes:
1. Change
<system.web>
<compilation debug="true">
To
<system.web>
<compilation debug="false">
2. Add the following <machineKey> section to the <system.web> section
<machineKey validationKey="AutoGenerate,IsolateApps" decryptionKey="AutoGenerate,IsolateApps" validation="SHA1" decryption="3DES"/>
Reference:
http://support.microsoft.com/kb/911722