Ausweisapp2-ce fails to start

I installed the ausweisapp2-ce on my Linux Mint 20.2 but every time I try to execute the program, the following error appears:

realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
No protocol specified
qt.qpa.xcb: could not connect to display :0
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the       application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl,   wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

Abgebrochen

I already reinstalled the programm and installed the plugin xcb, without any changes.
Thanks for your help!

EDIT: Using xhost + fixed the problem and now it starts correctly. Although, realpath: ‘’: No such file or directory still appears…