No issues that I'm aware of. My Dropbox client and the website seem to be working normally. What exactly are you trying to open; the Dropbox folder on your computer, the Dropbox client on your computer (icon in the system tray or menu bar) or the Dropbox website?
I am having the same problem - started last night. I can open files but not folders... Another friend was experiencing it as well as we share one of the folders.... If multiple people are having issues - is this not a site problem??