I believe this is an issue with the PHP version on your server. It seems there’s a bug that’s preventing the content import on servers running a PHP version older than 5.6.
For now, I would ask your hosting to upgrade you to PHP 7 (it will make your website faster, too). If they can’t do that, I would ask them to do 5.6 at the very least, as anything before that is no longer supported and vulnerable to hackers.
We’ll get this bug fixed in GPP 1.8.
Thanks!