“problems with your website’s HTTPS connection” but SSL certificate is ok

Hello,
the wordpress health checker keep telling me that I have security problems with the HTTPS connection. Is sais “Your WordPress address is set to use HTTPS, but your website appears to be unavailable when using an HTTPS connection”, but if you go to my website you can clearly see that the https connection is perfectly working. (I have activated a “coming soon” plugin but you can still see that the site is available and that it uses the https protocol)
I contacted my provider and they confirmed that the SSL certificate has no problem.

Not long ago I had this identical problem, then one day, out of the blue, it was gone. Now, just as it was gone, it is back.

Can you help me understand what’s going on?
I believe my site is safe, but I would still like to understand the reason for this reported problem.

Thanks a lot,
Sarah

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