Order Link (cart.php) redirection issue


We are having a very strange problem:

When using an encoded order link from our WP website (i.e. https://www.scaleuptech.com/de/myscaleup/portal/cart.php/?a=add&pid=12&language=english) a customer is sometimes brought to the clientage login page instead of being brought to the order form. This only happens once, after clicking the same order link again it works.

What can be the cause of this issue? I am seeing 302 redirects in the web server log file for the cart.php requests.

Any help would be appreciated!