The address was wrong in the config of aupeix-GX723, and was the address of Dell-DM061.
The reason could be this:
On the box, Dell-DM061 and Gramps-JujuLand had static address, but not aupeix-GX723
I always open Gramps-Jujuland first, as it's my principal computer.
The last day it syncs, I ought to open only aupeix-GX723, and with wifi and ethernet.
This was why I saw somewhere the address 192.168.1.22.
I disabled the wifi the same day and after, syncthing, as it the address was 'automatic', take the address given by the box which seems to be the address of Dell-DM061 (what I don't understand), so the address having been registered was an address which works because Dell-DM061 was not launched.
Yesterday, I launch Gramps-JujuLand and then the two other computers at the same time. And aupeix-GX723 tried to use this address which was not possible, as Dell-DM061 was using it.
Today, suprised by the fact that the address seems wrong (after your answer), I modify it in config.xml, and it works.
I'm not sure that it's the exact reason, but it's the only explanation I found.