Error executing agent

A strange error started to appear in my Domino server logs, stating an agent couldn’t run:

AMgr: Error executing agent 'SendReminders' in ‘hr\mydb.nsf': You are not authorized to perform that operation

Obviously this is an access rights issue, but it wasn’t obvious at first where the access issue existed. My agent Runs on Behalf of another user, mainly so that emails generated from the agent will appear to have come from that user.

I confirmed that the signer of the agent (me) and the other id (let’s call him user B) used to run that agent both had appropriate access in the security settings for the server. It was bugging me that the issue occurred only on one database and I started dreaming up conspiracy-theory-like explanations like the database is corrupted or the server document is corrupted.

Fortunately I know from past experiences it’s best to ignore those wild explanations, at least at first, and look for a simpler explanation (Occam's Razor). It then occurred to me to check the databases ACL and make sure that user B had access. User B wasn’t listed on the ACL. No surprise, as soon as I gave User B access the agent ran fine.