The offifial website www.lautoentrepreneur.fr has been redirected to URSSAF’s website. I have no idea whether they are experiencing some technical problems, but I will chase them tomorrow.
In the meantime, you can still register your auto entrepreneur business with URSSAF. Here is a link to access to the auto entrepreneur registration form direct:
https://www.cfe.urssaf.fr/autoentrepreneur/CFE_Declaration
Up-date on 12th January: All back to normal this morning. It must have been a technical problem.