I moved and had to change my network.
After juggling for reconfiguring my airport express station, I found out it was impossible to connect to them, even after a factory reset : they just would NOT appear in the Airport Utility 6.2 program that is meant to manage those things.
However, I could see all of them appear in the bonjour browser, so there is no network issues.
Faced with that issue, I saw some other people could not either use their Airport Express anymore. I am now using the old utility as they are, which necessitates a hack, but works perfectly.
However, I still don't understand if there are anything we are doing wrong.
That seems wrong that Apple gave no way to use its product. Has anyone be able to manage its airport express with standard Apple program ? I dont understand the rationale of what happens and must be missing some information..