Answer
Reset the failed message's state (logged in as IFSAPP or another admin account), then resave the scheduled/application task itself — resaving it while logged in as an account like IFSAPP changes the task's initiator to that account instead of the departed user's. Going forward, avoid tying a recurring scheduled task's initiator to an individual named user's license at all; running it under a stable service-style account (like IFSAPP, or a dedicated service user) avoids this exact failure mode recurring whenever a specific person's license status changes.