I made it.
I downloaded the certificate with firefox by playing the webradio clicking on the lock symbol.
Then I followed these steps:
“Installing a CA” response by M. Haase
like this (I renamed the downloaded pem-file):
volumio@volumio-music:~$ sudo cp unicross-ub-uni-freiburg-de.crt /usr/local/share/ca-certificates [sudo] password for volumio: volumio@volumio-music:~$ sudo update-ca-certificates Updating certificates in /etc/ssl/certs... 1 added, 0 removed; done. Running hooks in /etc/ca-certificates/update.d....done.
This was successfull. 
Before that I tried:
curl --insecure https://unifm-streams.uni-freiburg.de/unifm.mp3
However, it didn’t work. It gave me a an infinite amount unreadable ASCII-symbols as output. 
Excellent. I like volumio. This would never be possible with commercial internet radio receivers!