Ubuntu with Nix and home-manager: GTK file/save dialogs make applications crash: GDK_PIXBUF env var faulty?

I’m using Nix (not NixOS) on Ubuntu 21 with home-manager to get some applications/services. I noticed that in all GTK applications (for example, firefox), displaying the Open/Save dialog crashes the whole browser. In other words, I cannot save files anymore. These dialogs work if I don’t use ~/.xsession (managed by home-manager) but a regular Ubuntu GNOME session.

I suspected something in the environment was at fault, so I compared both env outputs. If I unset GDK_PIXBUF_MODULE_FILE, everything works again. The variable gets set to /nix/store/7ids0vgrzwdlgaijg0ai7lqk1nnr5nly-librsvg-2.50.7/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache on my machine.

Anyone have a clue what this error might be caused by? What’s this variable and why is it set?