Symptoms
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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)
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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.