If the app does not start, especially after an update, usually this is related to incompatible plugins. To fix this situation, put the app in recovery mode and open a Web terminal. Get a list of installed plugins:
npm ls 2> /dev/null | grep ep_

The two plugins ep_cloudron and ep_etherpad-lite are required, any other plugin might cause the issue. Uninstall other plugins one by one with:
npm rm <pluginname>

Then see if the app can start up again by running /app/pkg/start.sh. If the app starts up and is accessible normally, disable recovery mode again, otherwise try the next one.

Plugins which did not cause the problem can be reinstalled again with:
npm i <pluginname>