After mender-server 3.4 upgrade : device show up a few seconds then hide

We upgraded from mender-server 3.1 to 3.4 (we have about 400 devices in our database).
After upgrading, when we connect to mender-server, all devices show up a few seconds then hide.
When looking to browser network, we can see that :

Do someone have any insight of what we can do to fix this ?