I had precisely the same problem and, having tried all the solutions suggested in this thread without success, raised a ticket (on 30th January 2019) through the Dropbox Business Support Chat feature. This involved sending them a video demonstrating the exact steps that constituted the problem as you have described. I tried again this morning and the issue seems to have been solved and I have now been able to permanently delete all the items displayed on the Deleted files page. One further tip for you: If you want to permanenetly delete all items, scroll through the entire list (to load them to cache), then use Ctrl-A to select all (which highlights the entire page), then Ctrl-click on one item (to leave all but that item selected), then Ctrl-click on the same item (to reselect it), then click on the permanently delete option. Now works for me!
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