I received my 1DX3 yesterday along with two 128 GB Sony CFExpress cards. Under what conditions do you see the Error 70? I'll try to reproduce it.
If you shoot in High Speed Continuous in either mechanical or via electronic shutter and live view, while bursting the error happens. Not all the time but fairly frequently.
It also seems to happen more after transferring images via card reader. The last couple of times I re-formatted the card and the problem seems to go away for a bit.

























