I have the same issue. I need to use SteerMouse to get my Logitech MX518 to work, as it's the only driver that will allow me to use the mouse other than Logitech's horrible resource hogging drivers.
My solution came from this thread: BTT preventing applications from horizontal scrolling
which was to go into the Advanced Settings / General tab and click the checkbox to disable BetterTouchTool while the shift key is held down. I don't think I use BTT much with the shift key, but I wish I didn't need to do this.
@Andreas_Hegenberg Any idea why BTT is swallowing horizontal scroll events generated by SteerMouse? Is this related to the feature that tries to disable horizontal scrolling with the Magic Mouse?