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This error message appears when Mailspring was unable to save your Mailspring ID password to your system keychain or keyring.
Make sure you've installed `libsecret` through your package manager. Then quit Mailspring, reset its settings by deleting $XDG_CONFIG_HOME/Mailspring or ~/.config/Mailspring and re-launch the application.
Quit Mailspring and delete it's data in `~/Library/Application Support/Mailspring`. This folder may be hidden on your system - the easiest way to delete it is to open Terminal.app and type:
rm -rf "~/Library/Application Support/Mailspring"
Then, re-launch Mailspring and sign in to your Mailspring ID again.
Quit Mailspring and delete it's data in `C:\Users\<You>\AppData\Roaming\Mailspring`. Then, re-launch Mailspring and sign in to your Mailspring ID again.