Could not load Qt platform plugin in FHSEnv

I’m trying to get some proprietary software to run.
Executing their downloads the application and a bunch of vendored libraries into a directory.

I tried nix-alien on the application, but now I get

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, webgl, xcb.

Adding libsForQt5.qtbase to targetPkgs in the buildFHSUserEnv created by nix-alien didn’t help. Am I missing wrapQtApp? Though my understanding is, that wapQtApp is not necessary in a FHSEnv, since the QT libraries should end up in their “expected” location(?)