Hey Steve,
Your issue might have to do with GPU + OpenCL compatibility. I’m working on Late 2015 Retina 5k iMac with a Radeon R9 M395X card, and even though it’s a nice, new card, it doesn’t seem to be compatible with OpenCL, so I’ve been running into similar issues.
Here’s how I resolved it: Open Premiere, go to File > Project Settings > General, and in the Renderer drop-down, change that to “Software Only”. Confirm that change, and you’ll be prompted with a dialog asking if you’d like to delete preview files. Delete your preview files. Then try your export again.
Hope that works for ya! Ideally Adobe will address this in their next update — here’s to hoping!
-a