WP e-Commerce Error (plugin conflicts)

Hello WPMU DEV Community,

I have a bit of an issue on a few of my WPMU websites and I need to see if anyone can help me.

It is an urgent matter for me.

I have WordPress Multisite set-up and I am using these main plugins (they are all activated and are all at the latest version):

-Wp e-Commerce

-BuddyPress

-Maintenance

On every site I use Wp e-commerce and Buddypress together I get this error below.

Fatal error: Call to a member function add_cap() on a non-object in /home/nam1465/public_html/mall/wp-content/plugins/wp-e-commerce/wpsc-core/wpsc-functions.php on line 434

I really need help every second time I visit a page (dashboard and front-end) on my website it times out for a few seconds and then brings me to a blank page with the error above.

Is anyone able to help me solve this, My website is not live yet..but I can't really get it live until I sort this out.

Best Regards, Elisha