Service is not available now, probably your Plesk is misconfigured.
Contact Your provider for details.

In my case, the above message appeared just after I configured the firewall (iptables) on my VPS. After some research on the internet, I found that Plesk likes to have the port 8880 opened additionally to the port 8443.

Ports you need to open on your firewall for Plesk to stop complaining:

Port 8443

Port 8880

If you are using iptables, below is how you need to configure your ports for Plesk and Virtuozzo.

# Virtuozzo
iptables -A INPUT -p tcp --dport 4643 -j ACCEPT

# Plesk
iptables -A INPUT -p tcp --dport 8443 -j ACCEPT
iptables -A INPUT -p tcp --dport 8880 -j ACCEPT

That’s it. This fixes the error message caused by firewall.