Getting 'Unauthorized' flash when trying to integrate AWS runtime

Using the new Console for the first time in quite a while, and going to launch the CloudFormation stack for the AWS runtime integration, whether I’m currently logged into the AWS console or not (I’ve tried both ways), I get shunted back to the Dashboard page (on a new tab) with an ‘Unauthorized’ flash at the top.

Currently on a (paid) Basic account, with two pre-existing racks in AWS.

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