Uninstall and clear Registry did not work for me.
There is another way to fix the problem:
Go to “C:\Program Files\Notepad++” Right click “notepad++.exe” and “Run as administrator” Then you can freely make change of “File Association”Notice that, if you run notepad++ (or associal files) as normal, you will see nothing in “File Association”. But don’t worry, file association still work for you.
Every time you want to change “File Association”, remember to run notepad++ “as administrator” again.