This issue is resolved in 6.6.9, but you can clean house with the following instructions:
With rekordbox closed, open the Finder and click Go > Go to Folder... from the menu bar. Enter this in the pop-open window:
Inside you'll find the file BeatportSetting.txt - delete that file.
Press Command + Shift + . (dot) and hidden files will be shown; delete the .beatport folder.
You can press Command + Shift + . (dot) again to hide the hidden files again, then launch rekordbox and login to Beatport again.
With rekordbox closed, open the following folder:
C:\Users\<yourusername>\AppData\Roaming\Pioneer\rekordbox6 -- note that the AppData folder is hidden; enable hidden files in the Explorer preferences.
Delete the Beatport folder and the BeatportSettings.txt file, launch rekordbox, try logging in to Beatport again.