WHMCS provisioning triggering error: Invalid data or no response

I have installed WHMCS provisioning plugin in a WP multisite and the modules in the WHMCS install. I configured everything but trying to activate a pending order to create a new site, I see the error:
Invalid data or no response: The receiving plugin may not be activated at (mysite)

I've read in other support forums that the cause can be a plugin conflict or hosting firewall. I deactivated other plugins and created a rule in the htaccess to prevent a firewall rule (login in WP from WHMCS now works). Can you please look into it?