Your picture has helped me to figure out, what I am doing wrong. I have tried to reserve the lab again.
After reserving the lab, I was automatically redirected to the topology, which had shown the right IP address as in your screenshot. Then I had lost the Internet connection (going by train), and the topology was gone.
On https://devnetsandbox.cisco.com/RM/Topology, I have seen a note that the lab is "Currently reserved by me", but I did not know how to re-connect to the lab. Therefore, I had clicked the lab icon and I have reached at a topology without APIC-EM IP address:
Clicking the "Reserve" button did not make sense, since this will reserve the lab a second time.
Now, I have found how to re-connect to a reserved lab:
On https://devnetsandbox.cisco.com/RM/Topology, click on Reservations in the left pane and then on the green lab name:
This will lead you to the right topology and the APIC-EM IP address is displayed:
Okay, that was a newbie problem.
@yachwen: Thanks a lot for your helpful answer.