Hello,
The "Unknown error" is displayed when the server response is not a valid JSON.
In the browser console, the content of the received response is logged ; but it's only client side.
Please check that your website do not display PHP warning directly in web pages ; any content added by a third party plugin could make the JSON invalid.
You can also check in your HikaShop paymenbt log if you have any entry which will indicates an issue during the order confirmation.
If you have the possibility to reproduce the issue ; you'll see the data by opening the browser console (shortcut: ctrl + shift + i )
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.