Agent has been blocked manually. Unblock the Agent.

As it's easter weekend (at the time I write this blog) and it's a nice tradition here to hide some small items (often colored eggs or sweets) so other can find them, this story perfectly matches.

In Enterprise Manager 13c I had an agent with status

Agent has been blocked manually. Unblock the Agent.

Unfortunately In the Agents drop down menu, there is no (sub-) entry to unblock the agent. A well hidden entry!
After several attempts I was hinted where to look. It's in the "Setup" => "Manage Cloud Control" => "Agents" area:

There all agents are listed, and when selected the blocked one, it can be unblocked in the top action list.

I did not find a useful entry in the documentation (but that's probably my lack of search-foo). At least there is a Note EM 12c: How to Block or Unblock an Enterprise Manager 12c Cloud Control Agent if Agent Status is Shown as Blocked in EM Console or Emctl Status Agent Command Shows Heartbeat Status : Agent Is Blocked ? (Doc ID 1392601.1)