When we go to http://vrlsandbox.info/syncbox/whmcs/whmcsportal/clientarea.php/?action=productdetails and click on button "Login to control panel"
it takes us to http://stest1.synkbox.dk/customErrors/500.aspx however, the url behind button is http://stest1.synkbox.dk
Additionally if you try with http://stest1.synkbox.dk in browser it doesn't throw any error. This works super fine within WHMCS however with WP and your plugin we face the mentioned issue.
Your immediate assistance is highly appreciated. We have granted access to you, if you need anything else please let us know.