Here is the Autodesk Knowledge Network Article on the issue of the recent Dropbox update interfering with the Open dialog. Yes, it says AutoCAD in the title (and it has apparently affected some AutoCAD users as well), but if you read the body of the article, you will see that Revit has been included as well. As of 8/23, the AKN article indicates that Dropbox is aware of the problem and is working on a fix.
You can uninstall Dropbox until the fix is available, or make use of this workaround in Revit, if the workaround works for you.
David Koch
AutoCAD Architecture and Revit User
Blog | LinkedIn
