Copy to Kindle via USB never completes

This problem is consistent and reproducible with two Kindle devices running current OS versions: Fire OS 4.5.5.3 and Fire OS 5.3.6.4. I log on to the kindle and connect it to my windows 10 system via USB as a media device. In Dopus 12, in the left panel, I navigate to the desired directory on the kindle. In the right panel, I select directories and files to transfer to the kindle.

Some of the files transfer, but then I get a Dopus popup error:
An error occured copying xxxx. The system cannot write to the specified device (29). I click on abort in the popup dialogue box.

Then I see the Dopus Copying popup dialogue box. Status remains unchanged indefinitely. I have to kill dopus.exe to exit the dialog.

In the past, I have copied many files to kindle devices this way. The problem may be related to Kindle OS, but then again, Dopus should not hang.

Any way to resolve the transfer problem?

Thanks!

BTW, here are two screenshots of the problem.

Please try the suggestions and alternatives here: