Previously authorised email address no longer authorised

I’ve been using Mailpoet for a while without trouble, but tonight, when I want to send a time-specific email, the plugin is telling me my default email address is not authorised. I have checked my Mailpoet account – that tells me, unsurprisingly, the email IS authorised.

When I click the option to authorise it, I get the following error message:

An error has happened while performing a request, the server has responded with response code 0

An error occured when performing the request. Please try again later

I’m on WP 6.4 and MailPoet 4.35.1.

Any wisdom, please?

Thank you

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