Certainly this isn't a regular situation. Note that what you're manually typing is the https:// URL, which isn't the first URL the browser requests when running from a non-SSL site.
On a non-SSL site, in a browser that has never visited your Marketo pod before, the browser will request the http:// version and will receive a redirect from the Marketo server to the https:// version. On subsequent visits the browser will no longer request the http:// version for Marketo but will request the https:// version only.
Because of the distinctly different expected behavior between the first and subsequent visits, this specific case can be very difficult to troubleshoot, because the http->https internal redirection persists in Incognito/Private windows as well.
I would recommend you hard-code https:// (instead of the protocol-relative //) and see if you can repro the problem.
I will make this as the correct answer for two reasons:
- Going to the https:// location has fixed the problem for those for which it has occurred.
- I have reason to believe that our security platform, Sophos, which has been installed on each of the machines which have had this issue, may be interfering with the marketo files trying to load in the background.
Thanks for the update. There could indeed be a malfunctioning antivirus signature/heuristic affecting this. Can't see what policy it would be trying to enforce, though.