Cloudflare customers on a paid Cloudflare plan have access to a feature called a Web Application Firewall (WAF). If a visitor is getting that error message trying to access your site, that user is triggering a rule in the Web Application Firewall instructing Cloudflare to block that visitor. You can view and take action against that event by going to the Traffic app in the Cloudflare dashboard, where you can see which rule was specifically triggered by that visitor.
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