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See which process took the port with netstat -tnlp | grep 80
Ajenti V only supports NGINX.
Ajenti V requires NGINX, Ajenti does not.
"It works" - страница по умолчанию работающего Apache. Либо не удален, либо до сих пор работает.
Ajenti-V? Удалите Apache.
Is Courier authd running? Are permissions on the folder ok? Is SELinux disabled?
See /var/log/exim4/mainlog for more info
I'd speculate that you have an /etc/hosts entry which maps vn137.webnetvn.com to a non-public IP.
No, it's not required, e.g.: https://104.236.18.21:8000/
Panel opens up just fine at https://webnetvn.com:8000/