Problem:
Apps cannot be installed via UMC.
Investigation
root@snhucs03:~# nc -vz 10.200.43.30 80
Connection to 10.200.43.30 80 port [tcp/http] succeeded!
root@snhucs03:~# nc -vz 10.200.43.30 443 -w2
nc: connect to 10.200.43.30 port 443 (tcp) timed out: Operation now in progress
root@snhucs03:~# curl https://asnhucs01.snh2.schulen-hannover.de/metrics-node/metrics/ -v
* Uses proxy env variable no_proxy == '10.200.42.1,10.200.42.2,10.200.42.3,10.200.42.4,10.200.42.5...
* Trying 10.200.43.30...
* TCP_NODELAY set
Solution:
So proxy is not the point here.
Open port 443 on your firewall