Hi,
Thanks. So with the FTP access I found out that all the files of HikaShop on your website are either from April 2022 or older.
I think there is a permission problem with the files of your website, probably after a server migration or something.
What I would recommend is to rename the folders administrator/components/com_hikashop/ and components/com_hikashop/ to something else via FTP and then install HikaShop's latest version's install package. This will add back both folders with the files of the latest version. This is necessary as the modifications to be able to support that were added only last year so it's no wonder this didn't work on your website while it worked on my end.
Note that this is a big update as you don't really have HikaShop 5.1.1 on your website but HikaShop 4.5.0. Only the database has been updated and that's why the version number says 5.1.1 as that information is stored in the database. So make sure you properly backup, and do the proper tests as a big update can sometimes have big implications (like old view overrides not working anymore, etc).