I have been struggling with this problem since early 2018 where Word for iPad give me an error message in the middle of editing a document stored on Dropbox. "Can't upload - Something's wrong and your changes can't be uploaded."
The document is originally created on my Word for iPad. The latest occurence was a couple of day ago when I first created a copy of my document and the continued editing the copy. After a few edits the error appears. The walk-around is to save the document on iPad and edit it there and then later move it to dropbox. But that's not a long-term way to work, is it!
The environment I work with is:
iPad 4 running iOS 10.3.3 (which is the highest supported one)
Word for iPad - the free version which automatically is updated now on 2.11.1
Dropbox - the basic level also automatically updated and now on 86.2.2
If screenshots help let me know and I provide them and any other information needed.
I have the original document available and it has no secrets in it and I can gladly share it should that be required. The text is in Finnish so it may be hard to understand, but that is surely not the purpose.