Problem accessing the Back End

Hello, I have a problem for some time with a wordpress site that I manage. When I login to my website (as an administrator or editor) the top menu appears (the black bar above) with the administrative options, but it gives me a kind of 404 error and I can not access any of the editing options, only it allows me to log out (luckily) and browse the site, but nothing to access the desktop, create a page, etc … It strikes me that I can do edits when I connect via dial-up, but not via wifi or data, Really very strange. I already tried changing the Theme and nothing. Any ideas please?

WP 4.9 (Tle problem exist before 5.x where launched
Theme: Responsive Theme 3.14

The page I need help with: [log in to see the link]