I've also been having this issue with Dropbox and Project Sharing, though not all the time. What I've noticed is that whenever this error message appears, it has been preceded by what seems like a Dropbox syncing error. When working correctly, the Project File on Dropbox should be edited by the Working File when work is 'committed'. In the Dropbox file history, this will show as the Project File being edited by the user who owns that Working File.
Occasionally, however, rather than the Project File being 'edited' when it syncs to the cloud, it will be deleted and immediately replaced by a new file with the same name. Anyone who does not have a Working File open at that moment seems unaffected by this; they can open their working file, refresh their drawing, and proceed as if nothing happened. But anyone who happens to have layers or objects checked out at the time this hiccup occurs, will get the error you're showing the next time they try to commit their work to the Project File.
Sadly, while I've been able to identify this cause of the error, I haven't been able to figure out WHY Dropbox will sometimes sync via this "delete-and-replace" method rather than simply editing the file in place, so a solution remains beyond my grasp (which is why I'm on the forums searching this topic!). There is one user on my current project (the only PC user) for whom this Syncing error happens every time. For the rest of us, it happens rarely, but it seems no user is immune.