I’m seeing exactly the same issue after migrating from Mac/iTunes to Nucleus with Internal Storage SSD. After migration, Roon cannot load files stored in folders with names that include accented/special characters. Changing the folder name to eliminate the accented character fixes the problem.
Bjork, Kodaly Quartet, Jose Gonzales, Bela Fleck, Bernard Lagace, Janelle Monae, Jeno Jando are just a few of the problems I’ve found so far. I have no idea how many album folder names are similarly afflicted. Not sure how to search and replace every special character that causes this.
Folder names all created by iTunes automatically (most recently by OS High Sierra, although my iTunes music library goes back to 2001).