Welcome!
Are you trying to get the German version or the English version?
I know this works with the English sites so, if you need the German version, try it with yours:
1) First, look in your Applications folder and make certain that no older OS installers are present. They can hinder the process.
2) Open the German version for Apple.com and select "Support" from the links at the black top bar
3) When the Support Home page loads, enter "high sierra: in the search field. This is how it looks in English.
The two "suggested searches" appear automatically.
4) Click the one that reads "high sierra download." It will take you to a support page with full information and, on scrolling dow, the download link that gives you special access to the Mac App Store for the 10.13 upgrade.
⚠️ NOTE: Use Safari to download There are many recent reports that FireFox is not working for macOS downloads.