Error on admin side on simplexml_load_string

Warning: simplexml_load_string(): Entity: line 1: parser error : Start tag expected, '<' not found in (...)/www/wp-content/themes/catch-mustang/update_notifier.php on line 120 Warning: simplexml_load_string(): Access denied. Your IP address [213.251.189.209] is blacklisted. If you feel t in (...)/www/wp-content/themes/catch-mustang/update_notifier.php on line 120 Warning: simplexml_load_string(): ^ in (...)/www/wp-content/themes/catch-mustang/update_notifier.php on line 120 This is the errors I've got since this morning on the back-office of the website. It's not affecting the website, and I can't think of any new modification that can have an effect on it. Any idea where to investigate