Suddenly Can't Import Any File to iTunes

I'm trying to upload mp3 and audibook files and some PDF and ePub files through iTunes to my iPhone 8 Plus.
Everything is up to date. I had no issues few days ago doing this.

Now I cannot upload anything into iTunes, when I drag and drop it does not allow me to drop file it's just showing "not allowed" button✖ like when you are trying to drop file somewhere where it cannot go.

When I go through upload file/folder it just does nothing. Does not show any error or something it just doesn't react.

When I open file in iTunes it opens but never shows in iTunes library.

Any help with this? No idea what to try next. It's very annoying.
Thank you.