Why can't the robokassa connect to the server?

Question

CentOS

I have VPS Centos 6 from ddos-guard and ISP manager. In general, such a problem is that the robokassa simply cannot connect to the handler. The site on the Internet is available and everything works, but the robokassa seems to be blocked. What could be the problem? Maybe a firewall or something. The hoster himself says that they have no problems. Robokassa says they don't have any problems either. Maybe someone has come across this.