2015-09 Issue on
Mathematica's starting up
If Mathematica freezes on startup, follow the advice below.
The simplest fix is to delete the entire Paclets folder,
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
The bad paclet is no longer on the Wolfram server, so once it is
deleted the problem should not recur. Other "good" paclets will
download automatically when needed.