E-Mail from Gandi Support:
Dear user,
Gandi is evolving, and so is its security!
Security keys now use a new protocol. Keys registered before September
10, 2019, are no longer compatible and have been deactivated.
Therefore, we have removed your security keys: yubikey from your account.
To maintain a satisfactory level of security, we have enabled MFA via email
for your account.
However, you can re-register them in your administration console, in the
ACCOUNT application.
Please feel free to contact us if needed.
Sincerely,
The Gandi Team