Thank you for your reply. I would like to let you know that the problem was due to a conflict with the User Avatar plug-in. When this plug-in was deactivated, then the Visual Composer and the customization page of the theme were available again.
I have deactivated some plugins and now the theme is displayed without any problem. Just in case that I need in the future to install another demo theme, should I have to contact you or now the installation of demo themes is working properly?
