Usually this indicates a conflicted or duplicate file within the project folder, something that could happen through a failed Dropbox sync or the like. Make a zipped backup of your project’s .scriv folder, then root through it for any files with “conflicted” or such in the name (it may differ depending on the service you’re using to keep the project up to date between machines) or for multiple project .scrivx files (the one with the yellow Scrivener icon that you click on to open the project). Move out any of these to your Desktop or a temporary folder outside the project, then try opening the project again. If that works, you can re-import any document files that you may have removed to compare with the one still in the project, if it was a case of a sync conflict. In the case of multiple .scrivx files, I’d leave the one that is newer (you can view the modified date within Windows Explorer) and then go through your binder within Scrivener to ensure the structure and meta-data is as you expect.