Fai.me failed minimal install when GUI and non-free are enable

Hello,

I'll try to use fai.me to generate an iso with GUI and some package. 
After some tests, I think there is a problem with the simultaneous use of non-free and GUI enable (and may be backports enable to).

A minimal install with no GUI, non-free and backports enable work.
A minimal install with GUI and backports work.
But, with a minimal install with GUI, non-free and backports enable, there is a dependancy problem when fai install non-free package (linux-image-amd64: Depends linux-image-5.6.0-0.bpo.2-amd64 [...] but it is not going to be installed) 
You can see error.log and software.log at 1'20 and 2'28 in this virtualbox record output here: https://drive.google.com/file/d/1M5bYxdHOiw0DerFxNmjwigD8utpPkuuS/view?usp=sharing
or reproduce the error with the ISO defined below.

Have you ever had this type of problem with fai.me and if so, how did you fix it?

Regards,

Jean-Mathieu

> Your FAI.me job has finished in 87 seconds with success
> Link: https://images.fai-project.org/files/faime-2IQ7KHIO.iso (1.3 GB)
> 
> 
> Your web config is:
> --------------------
> type="install"
> partition="ONE"
> desktop="GNOME"
> suite="buster"
> keyboard="fr"
> addpkgs=""
> rootpw=""
> username="debian"
> userpw="debian"
> github=""
> repo=""
> cmdline=""
> options="BACKPORTS NONFREE"
> --------------------
> 
> If you are interested in more details:
> Your FAI config space:
> https://images.fai-project.org/files/faime-2IQ7KHIO-config.tar.xz
> Your image was created using this command:
> fai-mirror -C /etc/fai-buster-bpo -m1
> -cDEBIAN,DHCPC,DEMO,FAIBASE,BUSTER,ONE,BACKPORTS,NONFREE,GNOME,FAIME,GRUB_PC,GRUB_EFI,AMD64
> /tmp/mirror
> fai-cd -C /etc/fai-buster-bpo -g grub.cfg.install-only -m/tmp/mirror
> faime-2IQ7KHIO.iso
> 
> fai-mirror.log:    https://images.fai-project.org/files/fai-mirror-2IQ7KHIO.log