Website reports error 403 in 360 Monitoring with a firewall configured: Operation timed out

Symptoms

• Website frequently reports error 403 in 360 Monitoring or the below one:

  [28] Timeout was reached (Operation timed out after 30000 milliseconds with 0 bytes received)

• The website is up and accessible.

Cause

Restrictions of a firewall.

Resolution

Adjust the configuration of a firewall that is used on a server to pass requests from the necessary 360 Monitoring IP addresses:
https://monitoring.platform360.io/whitelist.php

Note: if there is the Plesk Firewall check this article for adjustments.