You could try taping over the pins on the TC. The Canon 1.4x III and 2x III refused to take a shot with an 800mm mirror lens I tried, but with the pins taped over it worked.
I say "it worked", the image quality of a cheap 800mm mirror lens is dire. With a TC on it's terrible .
IT WORKED! Much obliged sploo! The camera (Canon 5D) takes a photo as planned with the Kenko 2x TC and Sigma 600mm mirror lens mounted! I do not know anything about quality yet but I'll give it a try tomorrow. You saved my day!