Hubspot Sales Add-in won't launch: "is not configured for single sign-on"

Question

I've been working with Hubspot trying to get their add-in to work in web mail (outlook.office.com/mail).

When installing the app from their website I'm told it is successfully installed, but on launching it, I simply get a vague "failed to launch, contact your admin" (which is me).

When installing the app via deployment (admin.cloud.microsoft/?#/Settings/AddIns), when I launch it, I get a little more information: "failed to launch - [the app] is not configured for single sign-on". Hubspot insists this is "an external, Microsoft-related requirement - they would be our next best-step of support".

Any help is much appreciated.