[Résolu] Can't deregister a development site

Background of the issue:
I am trying to register my site, but it's gone from the WPML interface. I can't deregister it. The issue can be seen at: lien caché

Symptoms:
Nothing happens when I try to deregister and register again.

Questions:
How can I deregister a site that is no longer visible in the WPML interface?
What steps should I take if nothing happens when trying to deregister and register again?