Hi @christophe971
I do see you’re using a child theme of Twenty Thirteen at the moment, which includes (at least) some CSS changes to the mobile menu. Is it possible that there are other customizations.
One thing you could check is to switch to using the parent Twenty Thirteen briefly and see if the issue is still there?
You could also try disabling the plugins in your site to see if that has an impact. If it does you can then reactivate plugins one by one until the problem re-appears (and you’ll have narrowed down to which plugin is the cause).
Hi @jordesign and many thanks for your answer and suggestions.
I gave a look at the files linked to the child theme, and I found that the URLs of the mobile menu, defined in the header.php file, had been modified. I made the correction and everything is working fine. I don’t understand how this happened since this file has nod been edited for years…
So many thanks again.
Christophe