Hi,
We can start with a link to your store so we would be able to see the problem directly.
With some instructions (the different steps), we can reproduce the problem in your website and understand more from where the error come from.
But without so few information, we can't say nothing (and we can't reproduce it in our local websites).
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.