Unacceptable TLS certificate error on uncommon browser

We have a client/prospect that can’t get to our site when clicking on the email tracking link. He’s running Linux and using the Midori browser.

For example:

Gives his this message:
"Midori can’t find the page you’re looking for
The page located at “ue160.infusion-links.com” cannot be found. Check
the web address for misspelled words and try again.

Unacceptable TLS certificate
Try Again"

When clicking on the link to the page on our site that the tracking link redirects to, it’s fine, so it’s something in the tracking link.

Since I don’t have a good way to reproduce without bugging them, I’m hoping to find the cause and possibly a fix.

Some research pointed to possibly updating the CA Certifications on his machine, and he did that,but it didn’t change anything: sudo apt install --reinstall ca-certificate

I exploring the link a bit, and found that there might be some issues, per report here:

But I would suspect way more problems/reports if this were a real issue.

Anyone heard of a problem like this? Any other ideas? Would like to avoid problems like this.