SQL Server Database Mail - Why can't I run an execute against xp_sysmail_activate?

Question

SQL Server 2008 R2 Standard x64

I can't send e-mail from this SQL Server (out of MANY that are configured pretty much identically AFAIK). I have created the mail profile and account. However, any time I tried to send an e-mail with sp_send_dbmail (with my account - in sysadmin role) or Database Mail->"Send Test E-Mail", nothing happens. Nothing goes into sysmail_log. The SQL Server error below is generated:

 The activated proc '[dbo].[sp_sysmail_activate]' running on queue 'msdb.dbo.ExternalMailQueue' output the following:  'The EXECUTE permission was denied on the object 'xp_sysmail_activate', database 'mssqlsystemresource', schema 'sys'.'

Details

• msdb.dbo.sysmail_mailitems has a record for each mail
• The database mail log is empty (msdb.dbo.sysmail_log)
• msdb.dbo.sysmail_sentitems is empty
• I have run sysmail_stop_sp and sysmail_start_sp
• "is_broker_enabled" is set to "1" in msdb

How is the execute permission denied when I am part of the sysadmin role? Any suggestions on what could be happening?