Hello,
I forgot to change the name in the new file:
%@ Control Language="C#" AutoEventWireup="true" Inherits="CMSWebParts_Membership_Logon_LogonForm"
Codebehind="~/CMSWebParts/Membership/Logon/LogonForm.ascx.cs" %>
So the code was calling the original web part instead of the custom one...
It's working fine now.
Thanks for the help!