Practically overnight, the SQL Server agent that had been working for over a year failed on a server running SQL Server 2008 R2. When attempting to restart it, it fails immediately with the message
This installation of SQL Server Agent is disabled. The edition of SQL Server that installed this service does not support SQL Server Agent.
But it is a full edition, and as I've said, its been working fine for over a year. Perhaps on a related note, the same day the Agent first failed, we also started seeing:
"License Activation (slui.exe) failed with the following error code: hr=0x80072EE7"
I know slui is a Windows activation related service, could it be related to SQL Server suddenly thinking it's an express edition?
So, as it turns out, I received bad information from the server admin, and the installation is in fact an express edition. The admin just assumed it was the agent that stopped working, since the backups started failing. Turns out it was in fact not the agent. Thanks everyone for all the input.