Install Flash Player 11.9 on case-sensitive filesystem
As I posted on an Adobe forum post the reason is the following: The installation tries to access /usr/libexec/Plistbuddy but the correct spelling is /usr/libexec/PlistBuddy. You can execute the following commands in a terminal to solve the problem and successfully complete the installation:

cd /usr/libexec
sudo ln -s PlistBuddy Plistbuddy