Thanks for pointing to that info about the certificates. I ran into the same problem today and was able to resolve it by installing the now-correct certificates.
It seems Adobe is now using Google Trust Services, and for me the proper cert was:
https://i.pki.goog/1p5.crt (install the same way as from the linked post)
If this is not the correct cert for you, you can follow these steps (for Firefox on desktop, may vary for other browsers or mobile):
1. Go to
https://acs.cdn.overdrive.com (from the "Loading book from ..." log message in Calibre)
2. Click the "lock" icon at the left end of the URL bar, click "Connection secure" then "More information"
3. This page has 3 tabs, for me the middle one was the one I cared about -- "GTS CA 1P5"
4. Go to
https://pki.goog/repository/ (linked from the bottom of the middle tab), and scroll down to the "Subordinate CAs" section.
5. Find the cert you need and click "Action" to download the certificate format you need, then install it