Service account doesn't have rights to check domain user SIDs

We started getting the following error in SSRS on subscriptions executions

Failure sending mail: The report server has encountered a configuration error. Mail will not be resent.

I naturally started looking through the SSRS logs and found this wonderful gem:

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.;

So what am I to do? Well the good news is there is a KB article that helped point me in the general direction of what to look at: http://support.microsoft.com/kb/842423.

I created a test subscription (as advised from the KB article) that would execute within a minute which only created a link to the report not embedding or attaching the report. After successfully receiving this report it confirms that the AuthzInitializeContextFromSid API function call is the issue and the service account is having permission issues executing it.

The three methods to resolve this issue in the KB article didn't apply to me as I know that the SSRS service account I use has enough permission to run everything required as it was working before. So instead of following the directions I decided to restart the SSRS service instead.... and then... BAM! "Service cannot restart due to a LogOn failure."

At this point I re-entered the service account credentials, started up the service, sent a test subscription and everything works again. So super weird how Server 2008R2 forgot the service credentials but at least it was a simple fix.