I can’t help you on how to reset this specific setting but, paste this into your DefaultEngine.ini under [/Script/Engine.UserInterfaceSettings] and the curve should ok again.
UIScaleCurve=(EditorCurveData=(PreInfinityExtrap=RCCE_Constant,PostInfinityExtrap=RCCE_Constant,DefaultValue=340282346638528859811704183484516925440.000000,Keys=((Time=480.000000,Value=0.444000),(Time=720.000000,Value=0.666000),(Time=1080.000000,Value=1.000000),(Time=8640.000000,Value=8.000000))),ExternalCurve=None)