Thanks for the answer. For future readers I'll write down the necessary steps.

Open the PBX Admin UI, e.g. via myApps -> Devices -> select the PBX in the left part of the screen. In the first line of the menu select PBX, then in the second line select Objects. Enter the name of the app (or a part of it) in the input field next to "show", then click "show". Click the app name in the list of results, then click the Delete button at the bottom of the window.