Plesk is not accessible or goes on looping endlessly while accessing login page.
nmap utility shows that port 8443 is blocked on server IP:
# nmap 188.8.131.52 -Pn -p8443
Nmap scan report for example.com (184.108.40.206)
Host is up (0.093s latency).
PORT STATE SERVICE
8443/tcp filtered https-alt
Nmap done: 1 IP address (1 host up) scanned in 0.18 seconds
Port 8443 which is used by Plesk user interface is blocked by firewall.
Allow connection to Plesk user interface by adding appropriate rule:
# iptables -I INPUT 2 -p TCP --dport 8443 -j ACCEPT
Please also check KB #213932745 to make sure all necessary ports are opened to provide full Plesk functionality.