I thought I can use this direction vector as Quaternion-Axis. The cam should point than to the target.
But but it seems i need a different approach.
import c4d, math
cam = op.GetObject()
target = op[c4d.ID_USERDATA,1]
def main():
tarPos = target.GetAbsPos()
camPos = cam.GetAbsPos()
direction = (tarPos - camPos).GetNormalized()
angle = math.pi/2 #?
q = c4d.Quaternion()
q.SetAxis(direction, angle)
b = q.GetMatrix()
b.off = cam.GetAbsPos()
cam.SetMg(b)