One year after you gave us permission in your Amazon account by following this help article here, the authentication token expires. You should receive a notification from Amazon that the token has expired. If you didn't receive it, please address this with the Amazon support team as we have no influence on this. You can always check when the MWS access will expire in your Amazon account as you can see below:
In order to avoid delays in VAT filing due to missing data, you must redo your Amazon MWS settings in your Amazon account and in your vatjar account.
Go to your Amazon account to "User Permissions"
Here you must go to "Visit Manage Your Apps"
In case you need to enter again our developer Account Number you can either log-in to your vatjar account and go to the Integration menu and copy it from there or copy it from below: 0296-2190-5509
The last step is to enter your Amazon Seller ID and Authentication Token in vatjar in the Integration Menu: