@oscman
It is problem of backoffice manager plugin.
Go to index.php of this plugin and find this line:
echo '<li class="opt_del_user"><a href="' . osc_base_url() . 'oc-content/plugins/backoffice_manager/delete_user.php?user_id=' . $user_id . '&secret=' . $user['s_secret'] . '" onclick="return confirm(\'' .__('Are you sure you want to delete your account? \nNote that all your listings will be deleted too! \n\nWould you like to continue', 'backoffice_manager') . '?\')">' . __('Delete account', 'backoffice_manager') . '</a></li>';
Replace it with:
echo '<li class="opt_del_user"><a href="' . osc_base_url(true) . '?page=user&action=delete&id='.osc_user_id().'&secret='.$user['s_secret'] . '" onclick="return confirm(\'' .__('Are you sure you want to delete your account? \nNote that all your listings will be deleted too! \n\nWould you like to continue', 'backoffice_manager') . '?\')">' . __('Delete account', 'backoffice_manager') . '</a></li>';