Ränder lassen sich nicht ändern

Using Document setup to globally change margins will only work on pages that have the original Parent Page applied. Any pages that have a second Parent Page applied will not be affected by changing margins in Document Setup. Using Layout>Margins and Columns needs to be used to change the margins on any new Parent or Document pages that have the new parent applied. If you have the new Parent Page open and change margins through Layout>Margins and Columns then all pages with the Parent applied will be changed. If you only want a specific page changed then have it open and then use Layout>Margins and Columns on it. All other pages with that Parent will be unaffected.

You should change it in the Layout / Margins and Columns - with appropriate Master Spread active/open.