Please take the following precautions:
If the Audiobro Download Center (ADC) application is stuck, try these things in the following order:
2) Press Pause (not Stop) on the ADC. Quit the ADC, then Restart the ADC and resume.
3) Stop Download in ADC, Delete what was downloaded (we may have to reset your account), and then try downloading again from the beginning.
It seems like some Windows components that get installed alongside Kontakt 5 Player seem to resolve the blank-UI-issue for a few users. There is no harm in installing Kontakt 5.8 to make it work. Here is a link to a Kontakt 5.8 installer:
Kontakt 5.8 can work alongside Kontakt 6 without any issue (and LASS works in either). After installing it and restarting the Download Center I believe it will be successful for you.
(You may have to enable the showing of hidden files to get to the “AppData” folder)
That should completely reset your ADC.
You should now be able to log back in and download.
*if the user Library folder is hidden on your system you can access it in FINDER by holding “option” and using the “Go” menu in the menu bar. Or alternately, you can use Shift-Command-. (period) to show/hide hidden files.
(And it might be worth double-checking in Energy Saver that the “Put hard disks to sleep when possible” is not checked.)
Then try logging in and downloading again.