Yes, there was a recently pushed incorrect paclet update that will cause this startup hang. All platforms can be affected, not just Windows.
For a workaround, start a standalone kernel (
WolframKernel.exe on Windows,
WolframKernel in a terminal on Linux; on Mac you will need the full path to the kernel binary, typically a location like
/Applications/Mathematica.app/Contents/MacOS/WolframKernel) and evaluate
PacletSiteUpdate /@ PacletSites
which should allow a normal startup afterwards.
It is also possible to
after which things should work again, even if the network connection is left enabled.
As of this edit, the broken paclet is no longer on the server. For those still experiencing the startup hang, the steps above still work, but the easiest fix would be to either delete the entire
Paclets folder or just the
pacletSiteData_10.pmd2 file, which is located in the user base directory, typically under
C:\Users\<username>\AppData\Roaming\Mathematica\Paclets\Configuration on Windows
~/.Mathematica/Paclets/Configuration on Linux
~/Library/Mathematica/Paclets/Configuration on Mac OS X