Product not added to the cart

-- url of the page with the problem -- : harps.com.au/index.php/hika/product/96-mono-nylon-strings
-- HikaShop version -- : 4.2.1
-- Joomla version -- : 3.9.6
-- Browser(s) name and version -- : Chrome 76.0.3809.132
-- Error-message(debug-mod must be tuned on) -- : product not added to the cart

I get 'product not added to the cart' without changing any configuration when trying to add a product.
Hika shop was working fine up to today.

This is the URL:
harps.com.au/index.php/hika/product/96-mono-nylon-strings

This looks like it only happens on the browser that I am also logged into the backend with.
When I try it from Chromium or Firefox I can add the product.
I can also ad it from another machine with same version Chrome
What triggers this error?

Regards,

Thomas Boepple
NiagaraIT