Thank you for your help . I put the debug on, this is the only mistake I noticed today when I had activities that have disappeared :
[31-Aug-2017 16:40:43 UTC] PHP Notice: is_404 was called incorrectly. Conditional query tags do not work before the query is run. Before then, they always return false. Please see Debugging in WordPress for more information. (This message was added in version 3.1.0.) in /var/www/clients/client1/web20/web/wp-includes/functions.php on line 4139
Do you know if this can come from there? What to do ?