Products of different vendors

-- HikaShop version -- : 3.0.1
-- HikaMarket version -- : 1.7.3
-- Joomla version -- : 3.5.6
-- PHP version -- : 5.6
-- Browser(s) name and version -- : ie,firefox,chrome...

Hi,
I am using Hikamarket with different vendors.
Settings: ONE Vendor (every vendor gets paid himself directly)

When a user tries to add products from different vendors:
User cannot add a product from another vendor: that is as it should be

But there is NO alert message which tells the user that this is not possible.
There is only the regular popup which tells "your product was succesfully added...". Looks as the product was added correctly, but it was not.

Did I miss a setting in the backend?

Feel free to try:
products-different-vendors

Addition:
If deactivating "Add to cart Legacy"... this helps. Seems to be the Ajax part that avoids the right alert message.
Does this mean I have to use regular mode and not legacy? Always?