Problem with Internet Explorer 11 / Flexbox: Background-Image not shown

In Internet Explorer 11 the background-image is not shown. It is the background-image in main wrapper / startbild.jpg. In Chrome and Firefox it works well. I build this with Flexbox.

And also the W3C CSS Validator shows many errors of the Webflow CSS:
http://jigsaw.w3.org/css-validator/validator?profile=css3&warning=0&uri=http%3A%2F%2Ffairimmo.webflow.io%2F


Here is my public share link:
https://preview.webflow.com/preview/fairimmo?preview=e49450d8e46fb6dc992caa6fbb4a3a8f

The website:
http://fairimmo.webflow.io/

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