I figured it out.
The error occurred as I did not give a code for the coupon.
I thought this field should be empty as no code is needed.
With a code it works automatically – without giving the code in the cart.
Thank you so much for your help.
Thanks for fast reply.
This is exactly the settings I have used. And only those. But still it does not work.
I have disabled the Bulk Discount plugin as well.