RekordBox album art drag&drop doesn't work on 4K?

I just switched to a 4K monitor (instead of Full HD dual screen setup) and I can't Drag & Drop image files to load album art to MP3-files anymore. The crossed out circle appears. I can still load by manually clicking the Upload button and selecting a file. But it takes a heck of a lot more time now. Anyone familiar with this problem? And any one know how to fix it? Thanks in advance...