Can't delete a user

When I'm trying to delete a user, I get this error message.

Fatal error: Using $this when not in object context in /home/stefnitert/domains/regionofinterest.com/public_html/wp-content/plugins/buddypress/bp-groups/bp-groups-classes.php on line 1053

Does anybody know how this is possible? And how to solve this?
I'm using the BP 1.2.5 and WP 3.0 on this site http://regionofinterest.com

Thank you.