Strange "First Click Only" issue that is not redirecting in product page but in cart

Hello,
We have Wordpress 4 and also just upgraded the WHMCS Integration plugin to version
The problem we have is strange because it happens only on the very first click.
What happens is that when we clean the browser temporary (cache and cookies) and we click in a link like :

http://www.domain.com/permalink/whmcsportal/cart.php/?a=add&bid=6

or

http://www.domain.com/permalink/whmcsportal/cart.php/?a=add&pid=7

The first time that we click, we are not being redirected in the product page, but instead in the cart with all the product list.

It seems like it is "loosing" the part "?a=add&pid=7"

The strange thing is that if we return to the homepage and we click the link for a second time it redirects to the correct product page instead of the cart.

This seems like a cache related issue because after the first click it works fine, but I am not sure.

It is not a browser related problem because it happens in all browsers that we tested.

Any suggestion on what to try?

Regards,
Fabrizio