This error message appears when Mailspring was unable to save your Mailspring ID password to your system keychain or keyring.
Linux
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. Then re-launch the application.
macOS
Quit Mailspring and delete its 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.
Windows
Quit Mailspring and delete its data in C:\Users\<You>\AppData\Roaming\Mailspring. Then, re-launch Mailspring and sign in to your Mailspring ID again.
Happening Often?
If this is happening to you frequently, please vote for and comment on this issue, including your operating system and Mailspring version in your comment: Frequent "Your Mailspring ID is missing required fields"