Hi,
A warning is not an error ; so I don't think it's the real source of your issue.
I also want to add that you shouldn't display PHP warning messages in your website when you're in production ; as the PHP error messages which should just be stored in the log and not display to the user.
Otherwise for the registratino problem, please check that you do not have something in the Joomla messages.
If you got the same page pre-filled, it means that the registration has been refused/canceled due to a missing/wrong element or a plugin. For most of these cases, a Joomla message is added in the queue.
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.