Thank your very much for this reply!
I will look into using a default theme on a backup, but it is not a really viable option, as we rely much on our current theme.
The coupon itself is applied, yes, the culprit here is the cart.
Our website is online under http://www.wachtigall.com, maybe you can think of an error source, reviewing it.
Do I get you right about the second one?
It is a functionality to define the free products e.g. per min. spend-tick, right?
The problem here for us would be, that we have got several quantities in different products – can I apply multiple filters for one coupon, so I can add 3 free products for products 1 and 1 free for product 2 at the same time?