Hi,
So you should use the browser console in order to see the XHR call (for the upload) and get the return content.
Because the upload is marked as failed by the javascript library, the return content is not a json confirmation code ; because you do not have any error message display, it should not be a json error code either.
So it's possible that you have a PHP warning or a PHP error. But without the error context, it will be difficult to know the source (and to provide a fix).
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.