Hello Panos, after your help I deleted my subdomain with SoccerLife because after updating I was having trouble with virtemart I could not see any virtuemart. So I decided to install the bt_master-v3.0-joomla3-quickstart package because I'm genuinely very good at it.
1) I installed PHP 5.6 package because otherwise I was wrong;
2) I used Clean Template's Cache and delete the Cached CSS files;
3) I have reactivated the PHP version at 7.0
4) I controlled from frontpage and everything worked fine;
5) I upgrade to version 3.8.0 and the result was HTTP ERROR 500, both on the administrative side and on the frontpage.
Can it be solved?