Photoshop CS6's Adobe Camera Raw is 9.1.1, more than enough to directly read Nikon D800 raws. You do not need the DNG Converter.
You can download and install it here: Camera Raw installer for Adobe Photoshop CC and CS6
https://helpx.adobe.com/content/dam/help/en/camera-raw/installer/CameraRaw_9_1_1.pkg.zip ----> Direct Download for Mac.
CS6 stops ACR at 9.1.1. In the event you get a camera that needs Camera Raw 9.2 or later, then unless you move to Creative Cloud, you need the latest DNG Converter. 8.71 is really outdated.
Adobe Digital Negative Converter 9.x for the latest DNG Converter and Instructions for setting it up.
DNG 9.8 did not work on my MacBook Air. DNG 8.7.1 works fine on my new 27” iMac.
The problem is the laptop. Same CS6 and DNG on more than one iMac. No problems except with the laptop.
Remember, I stated that the problem came up all of a sudden after no changes. In fact I was able to convert part of an SD card and then it just refused to work on the following images.
After more tests, I cannot use RAW images made more than a year ago that were previously converted successfully.
I am dumbfounded. The laptop works fine in all other areas.