Hi @simplejimbo,
Do the “user sync” is a WordPress plugin or cpanel based tool?
WP Security > Firewall > Internet bots ban – Blank HTTP headers Ban POST requests that have a blank user-agent and referer this feature might conflict if the “user sync” have blank http headers and user agent.
If still an issue make sure that any of the below settings are on please try disabling it. one of the below features has conflict in such case.
1) REST API – WP Security > Firewall > WP REST API Can you please cross check the Disallow unauthorized REST requests:
2) Deny bad query string – WP Security > Firewall > PHP rules tab. Deny bad query strings: uncheck the checkbox and save
3) Advanced char filter – WP Security > Firewall > PHP rules tab. Enable advanced character string filter :uncheck the checkbox and save
4) 5g firewall rules – WP Security > Firewall > 6G Blacklist firewall rules tab. Enable legacy 5G firewall protection uncheck the checkbox and save
5) 6g firewall rules – WP Security > Firewall > 6G Blacklist firewall rules tab. Enable 6G firewall rules – uncheck the checkbox and save
Regards