Homepage isn’t displayed

Hello there!

I created a custom homepage for a new website, which is mostly based on custom html/css code. It works flawlessly as a normal page and is displayed correctly both in the preview and as a normal site. The theme is twenty-sixteen.

The weird stuff happens as soon as I try to set the page as my homepage. When I try to choose the site as my starter page (via Settings -> Reading -> Static Page), the page disappears and the custom blank blog page provided by WordPress is displayed.

I copied the page and gave it a different name for tests, whenever I switch the main page, the homepage transforms into this blank blog-pattern. You can check both pages here:

https://bettinawuelfingen.de/ (this is being displayed right now)

https://bettinawuelfingen.de/start/ (this is the identical code and *should* be displayed instead)

Any guesses?

Thanks so much in advance!

Hannah

The page I need help with: [log in to see the link]