Unable to open Plesk or it goes on looping endlessly while accessing login page

Symptoms

Plesk is not accessible or goes on looping endlessly while accessing login page.

A nmap utility shows that port 8443 is blocked on server IP:

# nmap 1.1.1.1 -Pn -p8443
Nmap scan report for example.com (1.1.1.1)
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

Cause

Port 8443 which is used by Plesk user interface is blocked by the firewall.

Resolution

Connect to the server using SSH and allow connection to Plesk user interface by adding an appropriate rule:

# iptables -I INPUT 2 -p TCP --dport 8443 -j ACCEPT

OR

# firewall-cmd --permanent --add-port=8443/tcp
# firewall-cmd --reload

Please also check KB Which Ports Need To Be Opened for all Plesk Services to Work with a Firewall? to make sure all necessary ports are opened to provide full Plesk functionality.