Could you please try to verify if the SQL user you used to configure the hub database (the database in Azure) on the sync group has been disabled or the password has changed?
On the Azure Database (the Hub database), make sure you provide the SQL Admin login with its current password as as explained in this article.
- Click on Azure DB
- Click "Sync to other database".
- Click on the sync group which you have created.
- Click on the Databases
- Click your hub server.
- Enter your username and current password.
- Try to sync.
On the section named "To add a SQL Server database" on the same article, please configure the Sync Agent correctly as explained on the article. On step #3, please provide the "sa" login and password of the on-premises SQL Server instance (not located in Azure). Review all steps on that article.
Hope this helps.