I used ultimate branding but now I found this code as error

I used ultimate branding but now I found this code as error what does it mean ? and how to fix it ? and I found it in more than one website ==> Warning: getimagesize(http://mamawebsite.com/wp-admin/images/wordpress-logo.svg): failed to open stream: HTTP request failed! HTTP/1.1 406 Not Acceptable in /home/mamaweb1/public_html/wp-content/plugins/ultimate-branding/ultimate-branding-files/modules/login-image.php on line 120

I have this problem in more than one site so i wish to know what is the problem so i can fix it in other sites .