I needed to repair the controls of my Canon 6D, after it received kitten damage. After re-assembly, I noticed that the middle column of AF points did not focus at all. They blink as normal when they start focusing, the lens makes one attempt to find focus and then quits. If I enable all AF points, the focus is achieved by the other AF points, never from the middle column.
The mirror and the focusing screen seem ok, I air-sprayed towards the AF sensor below the mirror in case that dust had accumulated, but to no avail. The mount contacts are ok and the issue appears with multiple lenses. I did not try to open the chamber of the AF sensor and I never touched any mechanical part in the mirror box during repair.
Can anyone suggest the source of the problem? What could probably cause the selective failure of only the center AF column, leaving the other AF points fully functional?