sudo certbot --apache
Saving debug log to /var/log/letsencrypt/letsencrypt.log

Which names would you like to activate HTTPS for?
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1: wiki.wsi.med.uni-muenchen.de
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Select the appropriate numbers separated by commas and/or spaces, or leave input
blank to select all options shown (Enter 'c' to cancel): 1
Requesting a certificate for wiki.wsi.med.uni-muenchen.de

Certbot failed to authenticate some domains (authenticator: apache). The Certificate Authority reported these problems:
  Domain: wiki.wsi.med.uni-muenchen.de
  Type:   unauthorized
  Detail: 138.244.128.5: Invalid response from http://wiki.wsi.med.uni-muenchen.de/.well-known/acme-challenge/JaXT4_09NpRX0cpvHQmKEh4N2i5GyeaXe8Y4q2nGMic: 503

Hint: The Certificate Authority failed to verify the temporary Apache configuration changes made by Certbot. Ensure that the listed domains point to this Apache server and that it is accessible from the internet.

Some challenges have failed.
Ask for help or search for solutions at https://community.letsencrypt.org. See the logfile /var/log/letsencrypt/letsencrypt.log or re-run Certbot with -v for more details.