Bought the Digital Download CE from Green Man Gaming on 7th July.

Was getting every Error (1, 2 AND 3) randomly when I entered my code. Mainly Error 3.
Contacted GMG - got the usual response of "Contact SE".

HOWEVER - after about 24hrs of trying, my code went through, no problems. Got my 20-digit, entered it into the Moogle Site, and Pre-Order complete.

I think the issue is more to do with SE than GMG.
Unfortunately, courtesy of so many variable errors, if there truly IS an problem with your code from the seller, it is getting buried under the SE problems and you wont know for certain until its too late.
Great Service already from Square Enix......