Hello @Lourens_Feenstra,
Can you please describe your setup in details (use this FAQ topic as a guide)?
We would also like to know more about music you are trying to add to Roon: is it an iTunes Library or you are trying to add a folder from your NAS as a Watched folder in Roon?
What is the file format of the music files which Roon couldn’t import (here you can find info about supported file formats) ?
Thanks for patience and apologize for inconveniences.