This article details one of the possible reason an authentication error can occur when connecting your Monzo account and the suggested workaround.
Bank accounts, Monzo
Relevant to users on:
Money Dashboard Neon
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The authentication error being returned may occur if there is a mismatch with the email address or if you have ever changed your email address on Monzo. Please try again and ensure you are entering the correct email address for your Monzo account .
Another less likely reason the authentication error code can come up is if you have ever changed your email address on Monzo. In this case, you would need to speak to Monzo to ensure your email address is updated.
Should the problem persist, please get in touch with the support team by submitting a help request here.