Backend errors after theme activation

If I activate the theme I get these errors on the top of the backend page: Warning: simplexml_load_string() [function.simplexml-load-string]: Entity: line 1: parser error : Start tag expected, '<' not found in /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-content/themes/catch-mustang/update_notifier.php on line 120Warning: simplexml_load_string() [function.simplexml-load-string]: Access denied. Your IP address [81.169.144.135] is blacklisted. If you feel th in /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-content/themes/catch-mustang/update_notifier.php on line 120Warning: simplexml_load_string() [function.simplexml-load-string]: ^ in /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-content/themes/catch-mustang/update_notifier.php on line 120Warning: Cannot modify header information - headers already sent by (output started at /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-content/themes/catch-mustang/update_notifier.php:120) in /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-includes/option.php on line 568Warning: Cannot modify header information - headers already sent by (output started at /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-content/themes/catch-mustang/update_notifier.php:120) in /mnt/webg/b3/05/52951005/htdocs/WordPress_03/wp-includes/option.php on line 569 This sticks in the backend of my fresh Wordpress installation and annoy the hell out of me. Changing anything in the theme's menu and pressing "save" gives me a white page with the same errors. If I change to any other theme the errors disappear, so the problem is a Mustang Theme bug for sure. Why do I get this and how do I get a bug-free installation. Swift help is appreciated!