There were no changes to XC, just as it is installed, applied the patch and the button stops working. Try any other skin and you will see it not working.
JFYI so you can fix that upgrade
1. Ideal Comfort - skin loads, button works
2. Books & Magazines - skin loads, button doesn't work
3. Light & Lucid - skin loads, button doesn't work
4. Artistic Tunes - skin doesn't load at all - error
Code:
PHP Fatal error: Uncaught exception 'SmartyCompilerException' with message 'Syntax Error in template "/var/www/blacksea/xcart450smarty/skin/artistictunes_business/customer/bread_crumbs.tpl" on line 5 "{if $location and not($main eq "catalog" and $current_category.category eq "")}" unknown function "not"' in /var/www/blacksea/xcart450smarty/include/lib/smarty/sysplugins/smarty_internal_templatecompilerbase.php:627
Stack trace:
5. Fashion Mozaic - skin loads, buttons doesn't work
6. Vivid Dreams - skin loads, button doesn't work