Tested on Mac and it working well. But you are not first who have this problem.
This is permission problem.
Give full rights to this directory /Users/YourName/.config/QtBitcoinTrader
Okay, so I can see how settings not getting written would cause this. ~/.config/QtBitcoinTrader doesn't exist, so am I supposed to change permissions on ~/.config/ ? Change to what exactly?
Also, this permissions issue is likely because this is not how preferences/application settings are supposed to be stored on mac osx. The correct directories would be ~/Library/Preferences or ~/Library/Application Support/