impossible for any user to register or login to my site

Dear all,

I have a mutli-language website (FR and EN). which allows for users to register and post ads.
I have no problem with the EN version but it’s is impossible for users to register/ log in fro the french version.

for registration there is the following error message:

Fatal error: Call to undefined function wp_is_mobile() in /homepages/2/d413313960/htdocs/avantpremiere/wp-login.php on line 67

for login there is this message:

Whoops! Page Non Trouvée
La page ou l’annonce que vous essayez d’atteindre n’existe plus ou a expirée.

Can you help me please?