Meanwhile it works on an interim basis
Since the Virtualhost section for jitsi was missing completely, I added it in /etc/apache2/sites-available:
# SSLCertificateFile /etc/univention/letsencrypt/signed_chain.crt
ProxyPass / http://localhost:8888/ retry=0
ProxyPassReverse / http://localhost:8888/
Regarding two aspects I am not sure how the final solution should be set up.
1.) univention-letsencrypt.conf seems to be autogenerated, so the jitsi part has to be configured (hard-coded) in /etc/univention/templates/files/etc/apache2/sites-available/univention-letsencrypt.conf ?
How can I re-generate the conf file from the template manually?
2.) For generating a letsencrypt SSL certificate, I have to add jitsi.domain.tld in LE settings. After that a virtualhost section in the above mentioned config file is generated, but for all apps - including wekan, nextcloud etc., but without jitsi, with the result, that jitsi points to UCS portal.
The missing jitsi line I can add by the workaround above, but how can I prevent these additional entries in univention-letsencrypt.conf?
Thanks and regards,