checkout-page does not work

hi
i am almost through in setting up my first woocommerce-shop using enfold 4.4.1 (child).
problem 1 ordering multiple numbers of the same product
i can write any number in the field but when i try to use the “+” button i can only add a maximum of 3
**********************
problem 2 being much more important at the momemt checkout page does not work at all
it only shows a blank (black) page with a small white rimmed square, no header/footer or any other field that would allow me to go back. i can use the back button, that will lead me back to cart page
what i already tried
set up a new page and inserted woocommerce-shortcode, this does lead to the same result
activated standard wp theme 2017, same result

hope someone will come up with an idea!
thanks n cheers
BIZ