HTTPS not working on Contact page

Hello WordPress colleagues,
could you please advise on the following:
The SSL protocol is not working on the Contact page of the website – checked it with Mozilla and Chrome on desktop and mobile device.
And we do not use any contact-form type of plugin.

Would you have a suggestion what would be the reason – as the others pages (tried only a few) are loading with httpS:// except this one?

Thank you in advance for your help

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