Hi,
Please let the patch that Nicolas gave you. In case when the issue will re-happen ; it should display useful information about the data HikaShop is trying to process.
Thanks to that, we will be able to understand from where the incorrect data is coming from and so, we will be able to make a good patch.
I made different test but I couldn't reproduce the issue.
I don't know if it is linked to a product ; if you need to login or to create a new account ; if you need to fill a specific custom field or not fill.. (even if it's link to the custom field because the error show up there)
About the HikaSerial issue ; the best will be to create a new thread in the HikaSerial section (or use one of your previous hikaserial thread) so I could get there more details about your current configuration and understand why the serial are no more generated.
An admin access might be required but in the first time, some information about your configuration and a sample order would be a good start.
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.