The makers of Dropbox informed us that the version of their interface (API) which was used in Notebooks 4 will no longer be supported after April 30, 2013. – If you are still using Notebooks 4.x and rely on Notebooks’ Dropbox sync, we would recommend to update to Notebooks 5 in time.

Users of Notebooks 5.x are not affected by this change.

Restricted Dropbox Support for Notebooks 4.x