Hi Stefan. I'm Greg, an installation specialist, 10 years Windows MVP, and Volunteer Moderator here to help you.
The scanner is not supported for Windows 10:
https://support.hp.com/us-en/document/c04635838
After plugging in the scanner and turning it on, check for Updates from WIndows Update to see if it will find a basic driver that let's you use Windows Fax and Scan.
https://faxauthority.com/windows-fax-and-scan/
If not as a last resort you can see if this basic driver provided for older scanners will work:
https://www.hamrick.com/vuescan/hp_scanjet_3400...
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I resolved your problem, I will keep working with you until it's resolved.
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