Quick update - I came across someone having the exact issue and it appears to be a problem with Dropbox. Has anyone else had this?
Like this chap, my Dropbox folder is my root project folder on my hard drive that I run Cubase projects out of. Will be contacting Dropbox but I don’t expect they’ll be able to give detailed advice on Cubase saving itself
The answer is in that thread. The file is in use. It’s not a problem with Dropbox or with Cubase, neither is designed to do that.
What is the advantage of it? You should use built in File History in Windows 10, which works perfectly.
I think the amount of overhead used to sync to a remote server, directly from the the project directory is certainly not worth it, even if it could work.