Fatal Error : require default-constants.php

Hello,

I’m french, please excuse me if there are any language issues in the following post.

I am in charge of a small website for my studies. The URL is masterisfdauphine.com .
It was created with the 1&1 app center tool.

I don’t know if the wordpress version was uptaded recently. FTP access to the metadata let me see the version is now 4.4.

The website crashed about two weeks ago. I get this message error when I try to open it :

Warning: require(/homepages/18/d439880397/htdocs/clickandbuilds/MasterISFDauphine/wp-includes/default-constants.php): failed to open stream: Permission denied in /homepages/18/d439880397/htdocs/clickandbuilds/MasterISFDauphine/wp-settings.php on line 22

Fatal error: require(): Failed opening required ‘/homepages/18/d439880397/htdocs/clickandbuilds/MasterISFDauphine/wp-includes/default-constants.php’ (include_path=’.:/usr/lib/php5.5′) in /homepages/18/d439880397/htdocs/clickandbuilds/MasterISFDauphine/wp-settings.php on line 22

What I tried :

– deleting all the plugins by FTP access. No change

– go to the mentionned file (default-constants.php) by FTP access and change the access rights to adfrw(644) since it was w(200). The site goes back online for a few seconds, but then my ftp connection is closed, and when I connect again, the rights are back to w(200).

What should I do ?

Thanks for your time.