error message

Wow, that's a strange error. No one reported such error yet. There is no reason that this error would come out from what we can see. It works everywhere else...

Which version of Joomla are you using ? Which version of HikaShop are you using ? Are you using PHP4 or 5 ? Windows or linux ? Apache or another web server ?

Could we have a back end access to the website to check your settings ?

I got the username and password indeed. It would be great if you could also give me the URL of your website

The problem is that the "RokNavMenu - Boost" and "RokNavMenu - Extended Link" plugins added by your template seems to overwrite custom menu options. Which means that the product selection was missing from the menu options in the database. Then, when HikaShop try to load the menu options, it doesn't find that option which should always be there and thus that error.

I temporarily unpublished both plugins and saved the menu and it worked.

This product selection option is the only menu option in HikaShop. All the other menu's options are handled with our own system in the menu Display->Content menus, so you won't have problems with these two plugins if you display your HikaShop products as a listing of product or listing of categories menus but only with product page menus, in which case you can unpublish these plugins before creating the menus and then enabling them again.

We will change a bit the code of that view to avoid crashing when that happen.

Now, for the fact that the product selection is not saved, that process is handled automatically by joomla. HikaShop just defines a custom menu option to display the dropdown. So it must be a problem between these plugins and joomla.

Do you have the same plugins installed ("RokNavMenu - Boost" and "RokNavMenu - Extended Link") ? These were the problem. You should try to unpublish them and recreate your menu.
Otherwise, you might have other plugins causing the same problem...