Cannot compose, select template, or move to any 'next' step

I am finding it impossible to 'compose' a newsletter from a WP post.
The first step is broken.
I cannot select a template from the dropdown - I have a Mailchimp template set up.
I am unable to do anything or move to next step.
I am using the latest version of the plugin and the latest version of Wordpress.
This plugin looked like it would be perfect - but this is a critical failure.

Any ideas what is going wrong?
I have seen another topic on this subject but whenever I visit that topic, I am no longer logged in, and logging in brings up an 'invalid token' error.