Hi,
If I access your HikaShop configuration on your backend, I get that error:
The HikaShop user synchronization plugin has been either removed or disabled from the website. It is a critical part of HikaShop and should not be disabled if you're using HikaShop on your website.Please enable that plugin via the Joomla plugins manager and then logout/login from the backend.
And I went to check in the Joomla plugins manager and I don't find the HikaShop user synchronization plugin.
So it has apparently been removed. Make sure that the plugins/system/ folder is writable and install again the HikaShop install package. It should add back that plugin and resynchronize all the users.
That's probably why you have issues there. I suspect that HikaShop doesn't see the user as logged in in some areas for some users due to that plugin missing.