Similar to Visvanath's suggestion: if this problem started recently then try to identify if anything has been changed on the site or server recently. The config changes report may help with this.
My first thought is that there's a problem with session handling. "Session" refers to the browser session and is how information about which user is currently logged in gets identified to the server, via a cookie sent to the browser. Is the cron being run? This removes expired sessions. Check there are no warnings regarding the cron under Site administration ▸ Notifications.
I mention the MoodleSession cookie because when I go to the root URL of your site, http://www.….com/, I get three cookies set: PHPSESSID, _subid and e73f7, but not the expected MoodleSession, I only get the latter at /login/index.php. None of these three cookies look normal for a Moodle site to me. If possible find out if these are expected on your server or site, they could indicate a problem.
It looks like your Moodle version is 3.5.4 or older. This is missing over two years of fixes and security updates. You should update to 3.5.18, maybe once the exams are over.
You should also plan to switch the site from insecure HTTP to HTTPS.
In case it helps others: The message Usted ya está en el sistema como WELLINGTON S.L., es necesario cerrar la sesión antes de acceder como un usuario diferente is You are already logged in as WELLINGTON S.L., you need to log out before logging in as different user; Inicio del sitio is Site home.