Try this:
- Click the link after File > Preferences > More preferences can be edited directly in the file. This should open the Arduino15 (or similar name depending on OS) folder.
- Delete all files in that folder except for preferences.txt. Please be very careful when deleting files from your computer. When in doubt back up!
- Try the Boards Manager installation again.
Pierre-Ernest:
I have searched around for a long moment on this site the way to write a new post
Then how did you miss the post titled "How to use this forum - please read.", which is a sticky on every forum section?
http://forum.arduino.cc/index.php?topic=148850