Hey @CotCot, welcome to the Community!
I hope you don't mind us replying in English.
Let's make sure the local files you're trying to import are using a format that Spotify can read.
Right now, Spotify supports .mp3, .m4p, and .mp4 files that don't contain videos. in case you're using .mp4 files, you must have QuickTime installed on your computer.
If the files are on the correct format and are DRM free, you shouldn't have any trouble importing them, so try reinstalling Spotify, or moving them to a different location to see if it helps.
Keep us posted.