I’m pretty sure it is not to do with the SSL certificate not being present. I think the robots.txt might be a misleading message too, since it looks like it is set up correctly.
I’m a bit confused as to why when I look through the page, there are 7 instances of twitter:card (some of why specify summary, some which say summary_large_image); and the images referenced do not seem to be accessible.
I’m surprised that you’re saying nothing has changed but that this has worked previously, as from what I’m seeing, there isn’t any code on the page that is likely to work.