I was recently working in a customer’s Office 365 tenant and was able to access all parts of the tenant except:

  1. The SharePoint Online Admin Center
  2. Any of the SharePoint sites
On another computer, everything was working fine.  I use my computer for all sorts of things and have to change settings from time to time, so I figured it had something to do with my settings.
To keep a long story short, I had to disable IPv6 on my wireless adapter.  I’m not sure why this made a difference, but as soon as I did it, the site came up instantly.
It’s possible this could be related to Hyper-V and all the virtual switches I have on my machine; I’m not sure.  I do know that I’ve had problems with other devices and applications due to the proliferation of virtual switches caused by Hyper-V.