We run into issue that whenever we used a Microsoft Dynamics AX configuration file with a Reporting Services instance (as mentioned in here: http://technet.microsoft.com/en-us/library/hh389774.aspx) the following error was reported the first time a report was run after the restart of the SQL Reporting Services instance:
"The DefaultValue expression for the report parameter ‘AX_CompanyName’ contains an error: Request for the permission of type 'System.Security.Permissions.EnvironmentPermission, mscorlib, Version=220.127.116.11, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. (rsRuntimeErrorInExpression)"Running the report for the second time, it renders without any error. The error is mostly seen with subsequent instances of Reporting Services because only then an Microsoft Dynamics AX configuration file (Microsoft.Dynamics.AX.ReportConfiguration.axc) is used but any Reporting Services instance that is set up to use an AX configuration file will throw this error.
The resolution we found was to use "FullTrust" permission set in place of "Execute" for the "Report_Expressions_Default_Permissions" in the rssrvpolicy.config but maybe there is some other solution that I was not aware of.