Try logging out of your Beatport account from within rekordbox, reboot your computer, then login again. If you still encounter and error, please file an inquiry ticket here and attach a copy of the Beatport log, found at:
Windows - C:\Users\username\AppData\Roaming\Pioneer\rekordbox\Beatport.log
macOS - /Users/username/Library/Logs/Pioneer/rekordbox/Beatport.log
...and our support team will gather some additional information to assist you in resolving this issue.