Thank you for your answer. Yes, I think I had already deleted the user (but I'm not sure because I can't see the name of this quarantined user).
So, I have re-installed Qlik and it is working fine now.
However, I have a question about this. My installation is connected with a Ldap. If we delete a user on the ldap before deallocate this user on Qlik we will have the same pb ?