I was finally hit by the Sync & GoTo failing after a Plate Solve, reporting that the target is below Horizon. (RainbowAstro RST-135 mount.)
ASIAIR failed in a similar fashion for both Regulus and Denebola, both of which were high in the sky and east of the Meridian (Regulus was just 30 minutes before Meridian crossing).
Interestingly, a GOTO works for both Regulus and Denebola, so ASIAIR does recognize that the objects are above horizon; but then ASIAIR contradicts itself when Sync & GoTo is clicked and reporting that the exact same object is now below horizon.
I then chose Dubhe, which is closer to the pole (and is a circumpolar star for me), and both GOTO and Sync & GoTo worked fine with that target.
Sync & GoTo was still failing even after Regulus has crossed the Meridian to the west.
Interestingly, if you turn on GoTo Auto-Center in the Mount Setup window, the ASIAIR will repeatedly plate solve and center the target that it failed to do with Sync & GoTo. But it is not always appropriate to let ASIAIR do auto centering (I leave it off most of the time since it stops centering before the target is completely centered).
Since ASIAIR does not complain when it was asked to do a GOTO, I decided to try a workaround, which proves to work. If you are flustered by Sync & GoTo bogusly reporting that a star is below horizon and refuse to send a GOTO command to the mount (or sending a bogus coordinate to the mount), try this:
1) issue a GOTO in ASIAIR as usual,
2) after the mount has done slewing, do a Plate Solve in ASIAIR, as usual.
3) Instead of Sync & Goto from the plate solve panel; do your own manual sync and GOTO by:
3a) click Sync Mount in the plate solve panel,
3b) click on GOTO in the GOTO panel again.
3a and 3b will move the mount closer to the target.
Whatever Sync & GoTo is doing, ZWO apparently has not tested it at all when releasing v1.5.3, in spite of numerous bug reports in the past months. I also don't see why step 3) above uses different code from the ones the ASIAIR uses to perform Step 3a) followed by Step 3b). Please fix the outstanding bugs before adding any new feature; they are piling up, with numerous bugs that involves bad celestial mechanics. Fix the coordinate system and most of the bugs will probably go away in one fell swoop.
Chen