We have received reports that users are not able to connect to Dropbox to manage their custom themes when they have changed their nation slug.
Update 3/28/17 -
In the instance of a slug change, it's necessary to right click the "Connect account" button, copy the link address, and then paste it into a text editor. Once there, you'll want to change the old slug name that will be in the URL string to the new slug, and then paste that into your browser. Please note that the folder created in your nb_sync folder for this nation will have the name of the old slug, not the new one.
Thank you Alex - we will let you know if there are updates to this we do not recommend that you change your slug.
As a workaround - in this case the customer got it to work by copying the oauth url and changing the 'state' parameter to point to (new slug) instead of (old slug).