public page not publicly accessible

The page referred to above contains a link (‘To subscribe, click here’) to a page (called Subscription) which contains a wpforms Contact form, through which a visitor may apply for access to password-protected pages.

What happens is that ‘Subscription’ opens with WordPress asking for wp-admin login. No-one can get into it as a simple user. I can see nothing in settings that differentiates that page from any other. Its visibility, for instance, is set to ‘Public.’

There are some password-protected pages, and they appear to work. Only the route to the password is blocked.

The page I need help with: [log in to see the link]