After experimenting with my mount simulator, I discovered that the Sync and GoTo bug in ASIAIR's plate solving is returning "Cannot GoTo: The current position of the target is below the horizon, can't GoTo" when the Local Sidereal Time is equal to the Longitude in hours west of Greenwich.
The bug affects every non-circumpolar star, and affects NexStar, SynScan and RainbowAstro mounts equally (those are the ones I tried -- it probably affects other mounts too, since it appears to be in the non-mount-specific part of ASIAIR's code).
All of a sudden, you will find that you cannot use Sync and GoTo for every star that is not circumpolar, 12 hours later, everything will work again; only to repeat the next evening. However, it is not a random event. It happens right when the Local Sidereal Time matches the west Longitude of your location. With locations that are East of Greenwich it happens before midnight Local Sidereal Time, and for locations that are West of Greenwich, it happens after midnight Local Sidereal Time.
Since it appears to be related to the Local Sidereal Time, the bug will not pop up at the same time every day. The time it happens changes by 4 minutes per day.
A workaround is to use Auto-Center, if you don't care about accuracy of the centering (ASIAIR's auto centering is not very useful since it stops before it is accurate enough), or use Sync Mount in the Plate Solve panel, followed by a GOTO in the GOTO pull out panel to manually perform the Sync and the GOTO steps separately.
Chen