I tried some exports today and was not able to reproduce the failure that you are encountering. I suspect that you are hitting an error case inside the code and it’s not cleaning up afterward.
Three things:
1) When you do the first export, when it finishes, the task toolbar icon should return to an empty circle (it also shows progress as a pie chart). Does it do that?
2) The second export – do you see a spinner in the task toolbar icon, or a pie chart? A spinner would indicate a stuck export.
3) Have you found any particular combination of images or export options either works 100% or fails 100%?