Multisite: Items disappearing from shopping cart when hitting "checkout" button

I have the same problem as described quiet often here on the community pages already. We have a multisite setup (with subdirectories), run on the newest WP and Marketpress Versions.

I first had the problem all over the store pages, that once you hit checkout all items in the shopping cart disappeared. After including the COOKIEPATH into the wp-config.php (as described here: it is working fine in the main store, but not in the multisite sub-stores.

Any help appreciated! BTW, I already deactivated all plugins! And the .htaccess is fine, too!