That is the older version of the Gemini, the new version has round DIN motor plugs.
Here is what the manual say's re Autoguider Error:
7.1.13 Gemini Displays “Autoguider Error”
•
This message indicates that Gemini received contradictory signals (RA+ and RAor
DEC+ and DEC-) at the same time. If this happens occasionally, it can be
ignored. Otherwise check the cable from the autoguider to the Gemini and keep it
as short as possible. Do not use a twisted cable; it could induce spurious signals
from one wire to the other.
PS: The battery is OK isn't it, they give all sorts of mischevious reading and errors.
Cheers