@clement1 One thing you can try is going to your tenant dashboard and going to…
Settings > Advanced (tab) > and disable the “Use a generic response in public signup API error message” toggle (under “Settings” section).
Be aware that this is toggled on by default for security reasons - anyone can try signing up with an email address. Using the “generic” error message would prevent any hackers from knowing which emails exists in your tenant.
I believe this should provide a better default signup error message to your users in the New Universal Login screen, let me know!