Occasionally, Gmail requires you to log in on their website in order to continue using IMAP clients.
Simple Steps
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Go to this Google Account login page, to see your current login status: Sign In page,
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If it shows you as already logged in, Log out - using the Google Profile Picture menu.
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Then, do login for the account once again: https://mail.google.com/
These steps of freshly entering your account credentials should help your account fix itself.
Please open a Support Request if this does not work for you, so our support team can help further.