By default the remote errors property in SQL Server Reporting Services is set to false, which means error messages in a multi-server environment aren't very detailed. By enabling remote errors, you will get additional error information returned which is more useful than this:
For more information about this error, navigate to the report server on the local server machine, or enable remote errors.
Option 1 to Enable Remote Errors - DB Engine
1. Connect to the database engine in SSMS and navigate to the ReportServer database.
2. Query the ConfigurationInfo table to get familiar with it. Issue the following query:
SET Value = 'True'
WHERE Name = 'EnableRemoteErrors'
Option 2 to Enable Remote Errors - Report Server
This option applies to SQL Server 2008 and newer.
1. Connect to Reporting Services in SSMS.
2. Right-click on the server node and go to Properties.
3. On the Advanced page, set the EnableRemoteErrors property to True.
Option 3 to Enable Remote Errors - Script
This Technet article also describes how to enable remote errors using a script, if you'd prefer to handle it using a RSS (Reporting Services Script), which is executed using the RS utility.