The same thing is happening to me as well.
I got on Adobe Support chat. After waiting and our and a half, a representative responded. I was transferred to 3 other people. That rep knew what she was doing. I allowed her to take charge of my computer. She worked around for about 40 mimutes and fixed the problem. She had to uninstall something in Creative Cloud and reinstall it. I was then able to download and install Extension Manager CC without any problems.