Hello !
We have found the cause of our issue: The HAproxy in front of our galera cluster had an incorrect setting: the check script checked database status on port 3306, when it should have checked on port 3304. We added the following line to our HAproxy configuration:
default-server port 3334 inter 2s downinter 5s rise 3 fall 2
Now our highly available passbolt installation is working perfectly, thanks for your help!