If you use the mouse to select an item in a drop-down combo box, for example the Minecraft version for an instance configuration, when you click on the selected (highlighted) item, the combo box return value doesn't return the value selected.
Pressing Enter from the keyboard, while the item is highlighted in the combo box results in the desired behavior of the selected (highlighted) item being set as the return value.
I have the same problem. I've nuked my Minecraft directory and reinstalled the Launcher with no luck.