Hey, thanks Skytrill! Acceleration Shift Viewport wasn't showing when 3d-view was active. I had to switch back to 2d.
Everything was already set to 0 except the first one, 1g head tilt, which was set at 1 degree. I set that one to zero as well, but I still think the horizon (the track/world) is moving when I drive over a curb or so.
This is the only simulator in which I get motion sick in VR. I've driven Assetto Corsa, RRRE and Dirt Rally for hours without a problem.
But I just love the real mirrors and the racing feeling in LFS so I'd love to get it working.