Hello,
This time I succeed to reach your checkout AND your backend, and I see now the issue.
And I'm sorry because I have a bad news, you had right the issue come from HikaShop but for now, it's impossible to get the root of the issue...
The other point is that HikaShop 3 are coming and with it, a new checkout process that will for quite sure solve this issue (because the process is totally different).
Can I advice you to switch to popup mode like before for waiting HikaShop 3 ?
Awaiting news from you about this suggestion.
Regards