Due to Twitter's changes, users with both new and existing Twitter accounts connected to Simplified have to reconnect following this workflow.
If you already reconnected your account(s) adding your own API Keys but are getting the "Failed to authenticate. Please check provided credentials and try again." error, it means the user authentication settings isn't properly set up.
To fix it, follow these steps.
Go to your Twitter Developer Portal.
On the left side menu, click on Project & Apps.
Select the option with your account's name
Scroll down to User authentication settings
Click on Edit
Check the settings and make sure you have the exact same options enabled, and that on Callback URI/Redirect URL you have added https://app.simplified.com/oauth