Dropbox cannot sync problem.

This problem was solved for me by following this article on Dropbox support:

https://www.dropbox.com/help/72

Of which the key details were:

Permissions errors occur more frequently when using shortcuts (Windows), aliases (Mac) or symlinks (Mac/Linux) to sync folders outside of your Dropbox folder.

Often you can resolve the problem by reverting the file's sharing settings using an administrative account. Here's how:

On Mac OS X
The easiest way to fix all Dropbox settings folder permissions in Mac OS X is through the Terminal application. You can run Terminal using Spotlight or find it in the Applications > Utilities folder
Once there, copy and paste the following lines into Terminal one at a time and exactly as it is written and press the return key after each line:
sudo chown -R $USER ~/Dropbox
sudo chmod -R u+rw ~/Dropbox
sudo chown -R $USER ~/.dropbox
sudo chmod -R u+rw ~/.dropbox

You may be prompted to enter your password. Enter the password you use to log in to the computer to permit the permissions change
If you've moved your Dropbox at some point, replace ~/Dropbox in the first two lines with the path to your Dropbox folder.

This command will grant your user account read and write access to all of the files in the Dropbox settings folder. After running these commands, restart your Dropbox and it should be able to sync happily again.