in case someone runs into the same problem:
I updated Etherpad on my UCS 4.4-8 instance to the version release a few weeks ago - a major update with big usability enhancements!
In my environment the new version wasn’t available. The internal IP address of the docker container in /etc/apache2/ucs-sites.conf.d/etherpad-lite-app.conf was wrong, it was 172.17.0.5 (which in my case is the IP of one of the UCS Dashboard containers). I had to change it manually to 172.17.0.2 (I found the correct one in UCR variable appcenter/apps/etherpad-lite/ip). After reloading apache (service apache2 reload) etherpad is available again.