This error comes from the error.php file of your template; it is not compatible with Joomla 3.7 or later.
Your main site's templates' error page is called because you are trying to run Kickstart in a subdirectory of your site. As I have explained and documented, this will cause problems because .htaccess files cascade through the filesystem.
As I have explained in https://www.akeeba.com/documentation/akeeba-backup-joomla/restoring-backups.html#general-guidelines:
Do not restore in a subdirectory of your main site. For example, if your site's root is in
public_html do not restore to
public_html/dev. The reason is that the
.htaccess files, which tell Apache (your web server) how to server your site, cascade. That is, Apache will read all
.htaccess files in all folders leading to the one hosting your site's
index.php file. This will cause problems with the restored site which you will experience as 404, 403 and 500 error messages or blank pages. These have nothing to do with our software and / or the restoration. It's how your web server works. Use a subdomain instead.
If you are restoring on a subdomain, make sure that the subdomain's root directory is NOT a subdirectory of your main site. This is the same as the previous paragraph, really. Most hosting control panel software default to using a subdirectory of your site's root when creating a subdomain. For example, if your site is
www.example.com and its root is
public_html if you create the subdomain
dev.example.com your hosting control panel will put its root in
public_html/dev. Therefore you will have the problem we described above. In this case ask your host what is the best way to create a root folder for the subdomain next to
public_html, not inside it.
So, create a subdomain for your testing and make sure that its web root is outside and next to your main site's public root. If you do not understand what that means or how to do it, please do ask your host. They can tell you how to do that with their site management control panel.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!