What is the output of running ls -l /srv/shiny-server/ in your system terminal?
The shiny-server service runs as the shiny user so if your app's root folder is not owned by shiny or some other user in a shiny-apps group you would need to allow rwx permissions to everybody (i.e. 777).
Also, check if a log file is generated at /var/log/shiny-server while the app is loading, in some cases, it gets deleted after the session stops.