After going through all the 'fixes' other users had tried (none worked - including many re-boots, etc.), I deleted my Dropbox as a last resort, and now there isn't an issue. Even after re-installing Dropbox, everything is ok - but don't know for how long. It appears that there are several 'add-ons' like this that can cause issues ... can take hours to figure out! MS is aware of the problem; and claims to be "working on it".