Oracle Grid Control: Agent Collection State Disabled by Upload Manager / How to resync/unblock a Grid Control Agent

Problem description: Grid Control lists targets for a server as DOWN and agent has an UNKNOWN state. If you are checking the status on the agent, you will see that your agent is running, but has a Collection Status of “Disabled by Upload Manager”: oracle@racdb01 /u01/app/agent/agent11g/bin # ./emctl status agent Oracle Enterprise Manager 11g Release [...]