Imgur certificate expired

it happens in all challenges, that all image that lead to imgur are somehow not showed.

And when I tried to open the image in new tab, which then redirect to imgur website, it said that the site certificate was expired.

I tried to add exception, and then get certificate. But then the page just showed ‘Page Not Found !’.

I don’t know if it happens only to me or anyone else get the same situation. I use ubuntu 16.04 and firefox browser. Can anyone tell me the actual problem that I had? is it because of the browser, the os, the imgur site, or others?