Once again I stepped in the trap:
This time I installed the quickstart package of ShopandBuy. I wanted to set it up with the plugin Booking System Pro, which workes with woocommerce.
I had problems to set up the plugin, some features were not available. The reason, once again - but still unexpected:
Setting up the theme, the user role of the one, who is setting up the theme, is set by default to subscriber, and not - as supposed - to admin.
To correct this fact, you have to create a new admin, to log in as the new admin, and change the role of the person to admin, who set up the system...