You might try going into your Users folder and creating a folder named My Documents. Then right clicking and selecting Restore Previous. Maybe it will replace the original one. If that doesn't work, try just Documents.
If you look at the properties for My Documents, on the location tab, there is a "Restore Default" option. Not sure if that will actually move the folder or just change its path. Maybe the Move button would help. You may need to find the current target, and maybe that is where the access problem comes in.
I suppose you probably have the option of changing the security settings on the Dropbox folder to gain access, but not sure.
Do you have a recent restore point you could restore back to?
I suppose I...