Hi,
It is a SEF problem.
When the url is called directly (not in a post), it is like the SEF parameters break everything.
Calling "/basket/" works but "/basket/task-step/step-0" not.
Calling "/basket/product/listing" display you a product listing (which is not logical at all).
Calling "/basket/?task=step&step=0" works (because the parameters are not SEF).
Well, what I can say is that, we will need to investigate more with some debug trace.
Because we don't reproduce your problem in our local website, we will need a FTP access to your website, so we can modify some HikaShop core files and see some debug trace in order to understand why wrong data is send to HikaShop after SEF.
(An admin access could be interesting too)
Which version of Joomla are you using ?
Do you use native SEF or an external component ?
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.