Hi,
If you have other "blank page error", you can activate the joomla debug mode and change the "error reporting" level (change "system default" to "maximum").
At this moment, some blank pages (error 500) would display the PHP error problem.
In your case, it was a error in an override email notification.
We removed the error line but there is still the "error reporting" level in your server. Please remove it after your tests.
Best 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.