Hi,
When the page is displayed with the parameter "popup=1" ; the value of the field "return_url" does not contains the current url but it contains the homepage url.
I think the problem is coming from the function "hikashop_currentURL" which has been modified for HikaShop 2.3.5 in order to add more security (and does not allow redirection to external websites).
What you can do for the moment is to edit the file "components/com_hikashop/views/product/view.html.php" and replace
$url = hikashop_currentURL('return_url');
By
$url = hikashop_currentURL();
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.