Now you have to resort to a rather weird workaround. Because there is a language problem. If you right-click on your note file in the German-language online OneNote and Export only an empty window appears.
Instead, set the language in the browser in which the notebooks are displayed (bottom left) via change language on English (United States) and try the export process again: right click on it, Export notebook. This gives you a zip file that you can download via Export download, transfer to the other PC, unzip and open. You can then change the language via Change Language back on German Germany) reset to default.
This export window remains empty in the German-language OneNote; You have to switch the language at the bottom left to English (USA), then it works
Small addition: With OneNote (formerly “OneNote 2016”), which can be run on all supported versions of Microsoft Windows and is part of Office 2019 and Microsoft 365, you would have to use File / Export find a direct way to export notebooks.