Cause
These errors can be encountered in your Location Fail Log for website uptime monitors when the monitored website is answering with a 403 Forbidden response towards our monitoring locations.
This HTTP code basically means that the website is denying access for these web requests, so the website is not considered UP/healthy by our monitoring system.
You can read more about what this error means here:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/403
For websites using reverse proxy services, such as Cloudflare, this error could be issued by that service and not your web server. For example, if you misconfigure your Cloudflare firewall or WAF, then Cloudflare can issue these errors towards our monitoring locations.
Fix
We suggest looking into your web server access/error logs to see why it is issuing 403 HTTP errors.
If your website is behind reverse proxy services, such as Cloudflare, we suggest looking into their firewall or WAF logs to see why these errors have been issued.