Hi,
If you are getting an error while export then You have to see the error description on SQL Server.
For Error Description in Error Log:
Log on to SQL Server, go to installation folder of SQL Server Reporting Services. Open the latest log file and see the error description.
EG: C:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\LogFiles
Hope this will help.
Thanks, Ranjitsingh R | http://mscrm-developer.blogspot.com/ | MS CRM Consultant