We have been getting a 401 error with the shipstation plugin, after having the connection work fine for the past year. Does anyone have a lot of experience using this plugin? https://marketplace.magento.com/auctane-shipstation.html
Did you contacted Shipstation? The module is the official one so you should be able to ask about that problem.
Can you try creating a new API user for Shipstation and try connection one more time ?