You need to update the MWS Auth Token.


First, go to Seller Central's Permissions page and scroll down to the Third-party developer section. Click the 'Visit Manage Your Apps'

You should see something like this:


Click the Enable button.

You'll then go through a couple of screens where you click next. (NOTE: the Developer ID may be different..

and


Then you will land on the Congratulations screen. KEEP THIS INFORMATION. Copy the text and save it somewhere.


Now go to Zonmaster and your Store's Settings page (under the Settings menu). Click on your store's name.



Click on the MWS Auth Token on the Settings page and paste in the new Auth Token from the Congratulations screen. Then save and click the 'Recheck' button. 



That should get you back up and running!