It sounds like your Nginx config for Blesta is almost correct but needs a few adjustments. To fix error pages, ensure the correct path is set and add internal; to prevent direct access. For API issues, ensure Authorization headers are passed with proxy_set_header Authorization $http_authorization;. Also, verify FastCGI parameters like PATH_INFO are properly set and consider temporarily disabling FastCGI cache if problems persist. Don't forget to restart Nginx after making changes using systemctl restart nginx.