Hello,
I have the same problem.
Previously, my site was on a site with Centos6 and PHP 5.4 and the script XML-Sitemaps worked correctly. I moved my site to the server with Centos7 and PHP 7.2 and this problem appeared.
The script starts working again if I enable the "Use CURL extension" option. But I do not like this solution to the problem. This spoils the statistics of visits. The script does not have a referrer and I cannot configure the rule in the statistics script.
Earlier you wrote that
your server doesn't allow local network connections via port 80 (http) or 443 (https)
I looked at the rules for IPTables. Is there a difference between these records?
Old server (XML-Sitemaps worked correctly) ACCEPT all -- localhost.localdomain anywhere
New server (XML-Sitemaps not worked correctly) ACCEPT all -- localhost anywhere
So far, I don't see a solution to this problem. It is a pity that in the settings of the script it is impossible to add a referrer or change the user agent, for example as bingbot.
Can you tell me what to do in this situation?
Thanks