You have to make sure the entire line is there, i.e.
MouseSensitiveName="Normal",MouseSensitivity=50.000000,LastConvertedMouseSensitivity=0.014174
The value of MouseSensitivity is not important, but the sensitivity is reset if it's not present when you change the LastConvertedMouseSensitivity value. The game will then remove the MouseSensitivity=50.000000 upon start.