White Space on top of the page

Hi WPMUDEV

on our site https://montessori-international.com I have a white space and can't figure out right now what is causing it and how I can prevent it from showing up. It is the body tag showing white and something is moving the header down.

Setting margin and padding did not change anything
setting line-height to 0px in fonts 13px/26px it reduces the space but it is still there.
when I disable it the bootstrap line-height kicks in with 20px;

Thanks for ideas

Kind regards

Andi