Fail2Ban is installed and enabled in Plesk.
When opening Plesk or a website or logging in to an email account via webmail from a specific IP address (for example, 203.0.113.2), the connection fails:
This site can’t be reached
203.0.113.2 took too long to respond.
From other IP addresses Plesk, websites and email accounts are accessible.
The following error message may appear in Plesk during some operation:
Request to backend API failed with error: Network error
The IP address 203.0.113.2 can be found in Plesk at Tools & Settings > IP Address Banning (Fail2Ban) > Banned IP Addresses or via CLI:
# plesk bin ip_ban --banned
Banned IP addresses:
203.0.113.2 plesk-panel Jan 1, 2020
The user's IP address has been banned by Fail2Ban.
Unban the IP-address or add it to trusted IP-addresses
To find out why an IP address got banned by Fail2Ban, see this KB article.