Creating a Facebook tab app using a Dropbox public link used to be simple, util Facebook made changes to it recently.
An unknown error "There was a problem saving your changes. Please try again later." will surface if you pump in the direct Dropbox link into the (Secure) Page Tab Url.
To solve this, you have to input into the Page Tab Url your Dropbox public link of the html file from
https://dl.dropboxusercontent.com/u/123456/foldername/index.html
to
This is accurate as of August 2013. Enjoy!