In testing server (outside studio) the logging system works as defined in logback.xml but when I want to see the in the UI (menu option: Server Log), the lookup field is empty. I can see the logs in the file system but no in the website.
How did you deploy the application on your server? Using WAR?
Path to the directory where the platform searches log-files is defined by the cuba.logDir application property.
Maybe you need to adjust it in local.app.properties (in tomcat/conf).
Yes, I deployed it using a WAR. In local.app.properties, the path was set to webapps/logs. So I created a symlink to /var/log/tomcat8 (standard dir for logs if you install tomcat8 through ubuntu package) and now I see them.