Sorry to add yet another topic on syncing problems, but I read the guides and searched the forums and still couldn’t find advice for my situation.
I primarily use Scrivener on iOS. I have been creating and storing all of those projects on Dropbox, not on my ipad. Few months ago I bought Scrivener for Windows and started importing projects from iOS. I was able to open the .scrivx files created on the ipad from the Dropbox folder on the PC.
However, some of those projects won’t open on either platform now. IOS says they are in “an older format” and shows an error message reading “unsupported project file format.” It wants me to open those on a desktop version. When I open the .scrivx files on desktop Scrivener, the program says it’s making a backup of the file, then closes.
Attempting to import these files into another project on Windows creates an empty project (draft, untitled file, research) in the binder. Transferring the .scriv “folder” out of Dropbox has no effect.
I don’t have any other syncing problems. New projects created on Windows open in iOS without issue, and vice versa. No unresolvable sync conflicts. The .rtf files with the actual writing content can be opened outside of Scrivener, as can the .scrivx file (in Notepad), so I assume it’s not corrupted. I’m really at a loss for what the problem is. 
I believe that this is a known issue with Scrivener for iOS,
Scrivener for iOS has to simultaneously support two project formats – v3 for the Scrivener 3.x projects created on the current Mac release and the current Windows beta, as well as v2 for the Scrivener 2.x on older Mac versions and the current Scrivener 1.x for Windows release version
If you create a new project on iOS, my understanding is that it defaults to a version 3 project – since that will be the eventual current release version for both desktop platforms, it doesn’t make sense to build a v2 project as a default and then require an update to the iOS version once the Windows v3 releases (as well as require current Mac users to have to upgrade the project format the first time they open it).
To test this, try downloading the current Windows beta version and attempt to open your projects there. If they open fine, you know your projects are simply in version 3 and you have two choices:
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Use the Windows beta version to work on them.
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Find a Mac you can use, or a friendly Mac user, and use the Mac version of Scrivener to export them back to v2 format. Move the current projects in Dropbox out of the way, let them sync out of iOS, and then move the down-converted projects into Dropbox. At that point, you should be able to open them in Windows 1.9.x and iOS without any further issues with project format.