I looked at Apache's error.log file and it reported the following two errors:
/var/www/enrico.io/htdocs/prestashop/themes/.htaccess: Deny not allowed here, referer: http://www.enrico.io/prestashop/
/var/www/enrico.io/htdocs/prestashop/modules/.htaccess: Deny not allowed here, referer: http://www.enrico.io/prestashop/
The .htaccess files in "modules" and "themes" is the same:
<FilesMatch "\.tpl$">
Deny from all
</FilesMatch>
I commented the "deny" line from both the files and the template loaded correctly. I then looked around about this issue and found it was reported here. I followed the suggestion by changing the .htaccess to:
<FilesMatch "\.tpl$">
Require all denied
</FilesMatch>
but it didn't work, and the error was again (this time with "require"):
/var/www/enrico.io/htdocs/prestashop/modules/.htaccess: Require not allowed here, referer: http://www.enrico.io/prestashop/
/var/www/enrico.io/htdocs/prestashop/modules/.htaccess: Require not allowed here, referer: http://www.enrico.io/prestashop/
so I simply commented the "deny" line again.