This is a known issue, with LR 2015.0, and Adobe didn’t have enough time to replicate and fix it with LR 2015.1, but probably in the next version in a few months.
The issue seems to be related to LR using multiple threads to export (as a way to increase performance) but somewhere one of those threads is getting in the way of another, and the result is partially-rendered images.
The only workaround I’ve seen is to restrict LR to 1 CPU/thread, somehow, and then there aren’t multiple threads to get in each other’s way.
Thank you for the quick response.
So does that mean that I get a refund on my month CC fees while the product doesn't work? After all I don't see how I can be expected to pay for something that doesn't work with the most basic function as exporting JPGs from RAW images
Specifically, the workaround on Windows is to create a Batch file (i.e. LR1.bat) with a text editor, and put the following text in it, and start LR by running the batch file instead of clicking on the shortcut. You could also create a new shortcut and have it run the batch file:
start /affinity 1 "" "c:\Program Files\Adobe\Adobe Lightroom\lightroom.exe"
This will start LR with only 1 CPU and avoid the issue.
You can also use Task Manager to accomplish the same thing on the fly by going to the Details tab, right-clicking on Lightroom.exe and choosing Set Afinity, then putting a checkbox next to only one CPU: