First go to Admin Tools, Web Application Firewall, Configure WAF. Make sure "Log security exceptions" is set to Yes; if it's not, set it to Yes and click on Save. Now try reproducing your issue. Immediately after that, please go to Admin Tools, Web Application Firewall, Security Exceptions Log. The latest log entry at the top should have the date and time of when the issue occurred. Please copy the Reason and Target URL here so that we can further help you.
Most common problem (which won't appear in the exceptions log): You have enabled Geographic IP Blocking (Geoblock) or you have blacklisted (Site IP Blacklist) the IP from which Facebook tries to access your site. Try disabling both features and see if it works for you. Otherwise a Web Application Firewall feature is being triggered by some of the URLs in your RSS feed. Facebook tries to access them all at once or on a very short period of time, therefore getting its crawler's IP address automatically blocked per the automatic IP blocking settings in Admin Tools.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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