"...got an error code 20....any ideas would be much appreciated..."
First remove the Sigma products from the 1DX. Remove the battery for a while. Make sure it is charged and reinsert it. Clear all settings even custom settings. Make sure you are back to default settings. Try the 1DX with a Canon brand lens (no off brand anything installed in or on the camera). ((Includes the battery))
Error 20 is not good. If what I suggested did not help call Canon 1 (800) 652-2666
BTW, I use that combo minus the tel-con all the time. I avoid tele extenders if at all possible and you should too. I am very careful to handle it by the lens only. Never by the 1DX. The big Siggy is just way too heavy to ever hold it by the camera. Did you do that while "chasing my dogs around the yard." ? The 1DX is a brute and very tough but even it has its limit.
If you need more lens consider the Sigma 150-600mm f/5-6.3 DG OS HSM Sports Lens. It is almost a twin to the 120-300mil. You will feel right at home with.
EB
EOS 1D, EOS 1D MK IIn, EOS 1D MK III, EOS 1Ds MK III, EOS 1D MK IV and EOS 1DX and many lenses.
