Hi,
It looks like an issue with Joomla 3.5.1 and the modification they made in the email system.
Fist, be sure that you're using the latest package of HikaShop 2.6.2.
Then, you can see to activate the debug mode or force the "error reporting level" to "maximum" in your Joomla global configuration. The goal is to catch the error message in order to know more about the source of the issue.
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.