@andy @ken I have some more info on this. it appears that the problem occurs when the main solver falls back to ANSVR during validation. SGP and PS2 are apparently talking about angles relative to sky N but astrometry appears to be talking about angles relative to sky S. the requested angle here is 357.5 degrees; when ANSVR solved the validation field it returned this string:
[2016-09-02 20:57:02] JOBS/cal: jstr: {"orientation":176.975426512331,"radius":0,"ra":311.920176057,"dec":28.9950780535,"parity":1,"pixscale":6.11484742075472,"epoch":"J2000"}
176.95 + 180 = 356.95, however somehow SGP has computed the rotator error as -497.5 degrees.
hopefully this is something to go on; this bug has hit me twice. it causes the center scope/rotate dialog to hang open forever and in the absence of meridian limits the telescope will just track forever right into the pier if it happens to be on the west side when this happens.