It rarely happens that a Twitter account becomes invalid or get disconnected from Typefully. You might notice this because a draft fails to publish.
To fix it, follow these precise steps in Typefully:
Log in Typefully with your usual account
Visit your account settings
Make sure you were logged into Twitter with the account you want to reconnect
Click on the 🔃 button next to the account you want to reconnect (which is failing)
You successfully completed to Twitter sign-in flow