If anyone still has this issue, I spent a few hours digging around. In my case, in the settings.cfg under AXIS_MOUSEWHEEL the name of the PRIMARY component was "None", this should be "Mouse ScrollWheel". Looking like this.
AXIS_MOUSEWHEEL
{
PRIMARY
{
name = Mouse ScrollWheel
axis = -1
inv = False
sensitivity = 1
deadzone = 0
scale = 1
group = 0
modeMask = -1
}