Running the flatpak fails: 'Failed to load module "xapp-gtk3-module"'

[LinuxOnTheDesktop]

On the newly released Mint 21 (Cinnamon flavour), the flatpak of Lollypop does not run.

$ flatpak install lollypop
Looking for matches…
Found ref ‘app/org.gnome.Lollypop/x86_64/stable’ in remote ‘flathub’ (system).
Use this ref? [Y/n]: 

org.gnome.Lollypop permissions:
    ipc        network        fallback-x11          pulseaudio            wayland
    x11        dri            file access [1]       dbus access [2]       bus ownership [3]

    [1] xdg-music, xdg-run/gvfs, xdg-run/gvfsd
    [2] org.freedesktop.Flatpak, org.freedesktop.Notifications, org.freedesktop.secrets,
        org.gnome.OnlineAccounts, org.gtk.vfs, org.gtk.vfs.*
    [3] org.mpris.MediaPlayer2.Lollypop


        ID                                          Branch       Op       Remote        Download
 1. [✓] org.freedesktop.Platform.ffmpeg-full        21.08        i        flathub        4.7 MB / 4.2 MB
 2. [✓] org.gnome.Lollypop.Locale                   stable       i        flathub        8.2 kB / 589.1 kB
 3. [✓] org.gnome.Lollypop                          stable       i        flathub       22.7 MB / 17.6 MB

Installation complete.
$ flatpak run org.gnome.Lollypop
Gtk-Message: 02:39:35.241: Failed to load module "xapp-gtk3-module"

[Eonfge]

I was on holiday so you had to wait a bit for a reply.

This sounds to me like a Cinnamon issue. For Flatpaks to properly work, they need certain components on the system. One of those is a file-browser extension so you can safely read and write to the disk. You're missing that for GTK3 applications.

Can you check if other GNOME applications work from Flathub?