"This connection is not private." - SSL Problem

I have a website that we recently installed an SSL Certificate on. It's been working great except for with some Safari users. For most users, the site now shows as secure and everything is fine. Over the past few days though I've gotten a few e-mails from people saying that they are getting an SSL Error that says "This connection is not private." I've had reports of it happening with Safari on a MacBook Pro as well as a few iPads. From what I can see, it seems to affect about 15% of my Safari users. I can't replicate the issue on my MacBook Air or my iPhone so I don't know what's going on. Has anyone else had this problem before? No problems for any of my Chrome, Firefox, or IE/Edge users. Here is the screenshot:

https://i.imgur.com/PHip4C3.jpg

The site is https://usufans.com/. I'm at a loss as to what could be causing the issue as everything appears to be fine with the certificate.