certificate verification "failing" message

Hello,

At https://validator.w3.org/, when I enter the address
https://pretextbook.org
I get a  message that

500 Can't connect to pretextbook.org:443 (certificate verify failed)

However, this seems to me to not be correct. All web browsers and wget
resolve the page without issue, including on several systems. And when I
follow the suggestion to confirm the domain records are correct at
http://dns.squish.net/, everything resolves.

Is it possible that the validator has cached some old certificate for this
domain? Or what else could be going on here?

Thank you.
Alex

-- 
Alex Jordan
Mathematics Instructor
Portland Community College