Error text:
Disabling dome slaving
Aiming scope at light source or screen
**Script Error (Tracking has been stopped)**
Source: ASCOM.GeminiTelescope
Message: Az value is invalid
Location: line 771 column 13.
Script Context:
768: End If
769: If DoFlatAz >= 0 And DoFlatAlt >= -90 Then
770: Console.PrintLine "Aiming scope at light source or screen"
=> 771: Util.UnsafeSlewToAltAz DoFlatAz, DoFlatAlt ' Position the scope (Synchronous)
^
772: Else
773: Console.PrintLine "Scope remains at current position"
774: End If
-- end --


Azimuth portion of AutoFlatConfig.txt:

DoFlatAlt -69.0 ; Altitude of flat panel/screen
DoFlatAz 220.0 ; Azimuth of flat panel/screen

Rummaged around the ACP and the Gemini WWW sites (gemini-2.com) but don't see anything that indicates a problem with these coordinates.