Mickaël, I am out of ideas as well 
The only one Idea I got left, is to re-check snap connections.
By default snap connections maas on the re-packed MAAS will be disconnected (most of them)
root@maas-danger:~# snap connections maas
Interface Plug Slot Notes
avahi-observe maas:avahi-observe - -
content - maas:maas-logs -
content maas:test-db-socket - -
hardware-observe maas:hardware-observe - -
home maas:home :home -
kernel-module-observe maas:kernel-module-observe - -
mount-observe maas:mount-observe - -
network maas:network :network -
network-bind maas:network-bind :network-bind -
network-control maas:network-control - -
network-observe maas:network-observe - -
snap-refresh-control maas:snap-refresh-control - -
system-observe maas:system-observe - -
time-control maas:time-control - -
Thats because when you install snap with --dangerous flag provided, autoconnect is not performed.
The easiest way to connect all required slots and plugs is to execute
snap connections maas | awk '$1 != "content" && $3 == "-" {print $2}' | xargs -r -n1 sudo snap connect
Then of course don’t forget to run:
sudo snap restart maas
With all plugs and slots connected it should look like his:
root@maas-danger:~# snap connections maas
Interface Plug Slot Notes
avahi-observe maas:avahi-observe :avahi-observe manual
content - maas:maas-logs -
content maas:test-db-socket - -
hardware-observe maas:hardware-observe :hardware-observe manual
home maas:home :home -
kernel-module-observe maas:kernel-module-observe :kernel-module-observe manual
mount-observe maas:mount-observe :mount-observe manual
network maas:network :network -
network-bind maas:network-bind :network-bind -
network-control maas:network-control :network-control manual
network-observe maas:network-observe :network-observe manual
snap-refresh-control maas:snap-refresh-control :snap-refresh-control manual
system-observe maas:system-observe :system-observe manual
time-control maas:time-control :time-control manual