Hi @jonas.juden ,
Sorry to hear that you're having trouble.
Are you happened to export the library with Autodesk Fusion 360 PCB format from 3rd party website, e.g. https://www.ultralibrarian.com/?

If this is the case, the exported libraries in that format has a same dummy URN which is incorrect. And then when you importing the libraries you will see the dialog you met.
As a workaround, please export the library with Eagle6 Library format instead. Which will download a script file instead.

Then you could run the script in Library editor to import the library without dummy URN. After that you could use that library without such problem.

More information about how to run script, you could refer to:
https://www.autodesk.com/products/fusion-360/blog/fusion-360-electronics-user-language-programming-u...
If it's not the case, do you mind sending the exported library file from 3rd party to us? You could attach it in this post if it's OK.
Sorry for any inconvenience.
Best regards,
Helen
Helen Chen
Principle QA for Fusion 360 Electronics