Just tried exporting to DNG for the first time in tPL 4.01 and it failed with an unknown error.
After experimenting with a lot of alternative versions of the image with and without various edits I found that the cause was the inclusion of Local Adjustments. I tested this on over a dozen other images in other folders and with other cameras and always got the same results. Adding a local adjustment caused the export to DNG to fail. Removing Local Adjustments resolved the issue.
To sum up, Local Adjustments caused export to DNGs to fail 100% of the time. Removing those Local Adjustments fixed the problem 100% of the time. No other edits, including Repair/Clone masks seemed to cause this problem. Can anyone else confirm my findings?