Hi,
I am trying to reproduce your error with "Registration + Guest" in the switcher.
I am using the latest HikaShop 2.3.0 build (updated few days ago).
When my browser pre-fill the username+password, I can't validate the guest mode because the script think that we started to validate the registration (because there is a password filled).
If I empty that field, I am able to finish the guest mode and continue the checkout.
So basically, I am not able to reproduce your problem.
I can't say that the latest 2.3.0 package will fix the problem, I see some fixes for the checkout but I can't guarantee that there is something for that specific part. But I know that Eliot worked on it end of last week.
If you want us to take a look, the best will be to have an access to your backend so we will be able to activate/deactivate the feature in a short time.
You can give us some "best schedule" and we will see to make our test in this period.
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.