While I was not trying to have both GNOME and plasma enabled, I was using both GNOME and Hyprland together for about a month, and had switched 1 or 2 days ago to the flake version of hyprland, see here.
What seems working to avoid this error from appearing is actually removing the flake version of hyprland, and disabling Hyprland:
programs.hyprland = {
enable = false;
xwayland.enable = true;
};
I am gonna try to see how to enable it again, but for now, I am okay with GNOME.
Thanks a lot for your precious help!