I was attempting to set up an email subscription on our production server the other day but kept getting errors when the subscription ran. The development server subscriptions ran fine, the configuration settings for both servers appeared identical.
The weird thing was that I could send a subscription through email as long as I didn't attach the report.
After much searching, and pulling out my hair, I finally found this KB article:
A call to the "AuthzInitializeContextFromSid" API function fails during the delivery of an e-mail subscription in SQL Server Reporting Services
My hint was the error in the error log:
ERROR: Throwing Microsoft.ReportingServices. Diagnostics. Utilities. ServerConfigurationErrorException: The Report Server has encountered a configuration error; more details in the log files, AuthzInitializeContextFromSid: Win32 error: 5; possible reason - service account doesn't have rights to check domain user SIDs.;
KB article 842423 walks you through how to give the account the permissions it needs to read the SID values.