Newton asks you to sign in again when your Newton sign-in on a device ends, and asks you to reconnect a single email account when that mailbox needs it. Start by checking which one you're seeing.
Try these, in order
Check what Newton is asking for.Your session has expired means your Newton sign-in ended: click Sign in again. This account is out of sync, or Authentication Required in Settings › Accounts, means one email account needs reconnecting. That's usually because its password changed or its access was removed in Google or Microsoft.
Did you reset your password? Resetting your Newton password signs you out on every device. Sign in again with the new one.
Has it been a month? Each device stays signed in for 30 days from when you signed in, then asks you to sign in again.
Sign in the way you signed up. If you created your account with Google, Microsoft or Apple, use that button. Signing in with an email and password instead shows This account uses Google, Microsoft or Apple Sign-In. Please use that button to sign in.
On Linux, set up a keyring. Newton keeps your sign-in in your system keyring, such as GNOME Keyring or KWallet. Without one, Newton shows Secure Storage Unavailable and asks you to sign in after every restart. Install or unlock a keyring, then sign in once more. If you use the Snap, update Newton first: a fix for Snap sign-ins has shipped.
Still happening? Send us your logs so we can look.
Good to know
Signing in again doesn't remove anything. Your email accounts and settings come back from your Newton account.
Changing your password, rather than resetting it, doesn't sign out your other devices.
Sign in with Apple isn't offered in the Android app.