I'm not sure what might be causing your problem.
I've just created a test site on my own web space with a Juicebox gallery page, added it as a Search Console property, verified the site ownership and run Fetch as Google , which, according to the Google support page "enables you to test how Google crawls or renders a URL on your site". No errors were reported, so I've been unable to replicate the problem you describe.
Maybe the troubleshooting tips in the Fetch as Google support page will help.
Perhaps the problem is due to the use of https://. I notice on the support page:
Fetched URLs are limited to the current site: for example, if the current Search Console property is to http://example.com you cannot fetch a URL from https://example.com or http://m.example.com.
Otherwise, as the error message contains 2 separate references to the server ("your server requires authentication to access the page" and "or because the server blocks Googlebot's access to your site"), maybe your web host can shed some more light on the problem for you.