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.
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%?