class-wp-hook.php:298 calls WPSS_Security::check_request_method()
class.security.php:701  calls WPSS_Utils::get_ip_addr()
class.utils.php:173 calls WPSS_Utils::get_web_proxy()
class.utils.php:686 calls WPSS_Utils::get_option() is $web_host_proxy is empty
class.utils.php:69 calls WP_SpamShield::get_option()
wp-spamshield.php:8087 calls rs_wpss_update_session_data()
wp-spamshield.php:2324 calls WPSS_Utils::get_ip_addr() again if not an admin or no session detected
and the whole loop starts over

Only when connectiong via IPv6, IPv4 works fine.