elpajare impossible for me to stack more than 3 frames of "Cleopatra's eye" at 21º altitude.
Yeah, assuming there are no tree branches sticking into the frame, it definitely sounds more and more like lack of understanding of atmospheric refraction by the ASIAIR stacking routine.
Also, I suspect that in this example they are trying to register the 4th frame against the 1st frame. The proper way for stacking is to incrementally register (i.e. 2nd frame with first frame, 3rd with second, 4th with third, etc...).
That being said, at that low altitude, even if it succeeds with stacking, you might see something that looks like star trails, anyway. With changing star colors, since atmospheric dispersion is also largest when atmospheric refraction is large.
Try setting your mount's hand controller to use King's Tracking Rate instead of Sidereal Rate. It may help a little. Most decent mounts have King's Rate in addition to Sidereal, Solar and Lunar Rates, even though the ASIAIR won't show it. Perhaps ZWO is afraid of confusing the 1,2,3, simpletons.
Chen