Try turning off Autofocus Microadjustment (AFMA) (see page 200 of the EOS 6D MkII Instruction Manual, or 5D MkII manual).
If AFMA is enabled, even if the adjustment for the particular lens/TC combo mounted is "0", some Kenko TCs will lock up the camera. This still occurs even if the lens is set to 'Manual Focus'. It's been fairly well discussed in certain forums.
I've got an older Kenko C-AF 2X Teleplus Pro 300 DGX bought in 2011 and all of my cameras from about 2012 on have this issue. With AFMA set to 'Enable' if the shutter button is fully pressed the camera throws an 'Error 01' and locks up the camera. It must then be turned off with the battery removed to reset the camera. With AFMA set to 'Disable' there is no 'Error 01' when using the teleconverter.