I have this same issue. I have disabled download permalinks as you suggested.
After this is done, when I first click on a category folder – nothing happens – the page seems to simply re-load. When I click on the folder a 2nd time it successfully expands out and I can see the files within.
Is there any reason it fails to work on the 1st attempt?
This is odd. I noticed that it works consistently on my mobile device (iphone or ipad) when I click on the folder icon, but when I click on the “link” (the category) that is when it doesn’t work correctly.