Hi,
Are you using a SEF extension ?
Because the tree is using an ajax call and replace dynamically a tag by the product id that you're expending.
But if the tag is not put "as it" in the url, (the tag contains curly brackets), the javascript can't perform the replacement and HikaMarket will not receive the category id during the ajax call.
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.