Dear Gryffindor,
As Aleks correctly pointed out, the problem in your calculation is that the SCF does not manage to converge. Particular attention has to be paid when dealing with spin polarized calculations: you should monitor in your output file how the spin density eveolves during the SCF procedure.
If you look in your OUTPUT.d12.txt file, you will see that at CYCLE 0 the spin density on your cobalt atoms looks like this
TOTAL ATOMIC SPINS :
3.0000000 -3.0000000 3.0000000 -3.0000000 3.0000000 -3.0000000
And it is totally coherent with your antiferromagnetic input setting. But at CYCLE 2 it looks like this
TOTAL ATOMIC SPINS :
0.6795143 -0.5759257 0.5922359 -0.6379062 0.6630575 -0.6747715
This means that the spin setting you wanted to obtain is already lost, the spin density on your cobalt atom is close to zero and the calculation is not going towards the solution you would like.
One thing that can be done is incresing the FMIXING, which helps mantaining your input spin setting. Furthermore, SPINLOCK and the DIIS convergence accelerator (present by deafault) do not go well together.
From the file I attach (in which both input and output are reported), you can see that if FMIXING is set equal to 95 and if the DIIS accelerator is activated only when the SPINLOCK is disabled, your system SCF procedure converges smoothly in 30 cycles. Your can try to optimize the geometry with these settings.
Hope it helps,
Eleonora