Occasionally, there may be instances where the Publishwide Sync App will freeze during the process of logging into Amazon. This is often due to 2-step verification not being enabled for your Amazon account.
To fix this:
Go to your Amazon account page
Go to Login & Security
Click on the Two-Step Verification (2SV) Settings
Click Get Started to enable your 2SV settings
The reason you may need to do so is because the Publishwide Sync App has a very specific set of actions that it will do to authenticate you with Amazon.
When you do not have 2SV enabled, Amazon may prompt other ways for you to prove that it is you who is authenticating with Amazon's servers.
But those other ways usually fall outside of the expected behavior by the Publishwide Sync App, and as such, the app will freeze up because it has no idea how to proceed forward.
Enabling 2SV will bring everything back within the expected behavior, and you also gain the additional bonus of securing your account. Double win!