Thank you, ts678. Problem solved:
I am using bigger blocks (500MB), so I have less files in Sharepoint.
I am using “Microsoft Sharepoint v2” now. It was difficult to configure because I have a special document library, not the default one.
I thought the “Destination path” should be “frank.sharepoint.com/sites/Duplicati” or something like this. It works using the “site-ID” and only a short path, look at the screenshot.
This was helpfull to find the “site-ID”: sharepoint online - How can I get the siteId of the current site with Microsoft Graph API? - Stack Overflow
Duplicati often causes small problems, but overall it is a great program!