specific WHMCS URLSs wont work

HI you awesom plugin creators,

We are testing your plugin and found a problem. We are trying to link to a specifick product page. When trying to connect to a redirect or specific WHMCS url we get the "Sorry this doesn't look like a WHMCS site at" error message.

The Thing is if you first visit the [wp-content] page (which works fine) and after that paste the url off the cart in the browser it works fine. But going directly to any of the cart pages without first visting the basic [wp-content] page it comes with the error.

is there an known subject about this? Can you please tell us how we could fix it?