I saw it in Chrome, with f12, after try subscribe. The url error is:
https://a21escolarab.es/index.php?option=com_contentnotifications&task=follow.subscribeGuest&redirect=aHR0cHM6Ly9hMjFlc2NvbGFyYWIuZXMvYmxvZw%3D%3D&component=com_content&objecttype=category&objectid=8&username=asfdasfd&useremail=mauromilla%40hotmail.com&e2388eccc5b06910e192c34961906f12=1