Beat,
Thanks - I have scanned everything I could find on this including the article you kindly provided.
The issue is that I've redone the certificate three times and am assured by A2 Hosting that LetsEncrypt is valid.
Also, the cert works for DT Register on the same website.
That screencap is from just a few moments ago in Auth.net and there are other settled/unsettled transactions there from the last few days.
I'm on hold with A2 to verify the cert again although it's simple enough to install.
Also, this message just started showing up after the last build.
Any other thoughts?
UPDATE: I'm switching certificates on the hosting to a paid cert instead of LetEncrypt to see if that makes a difference.
Mark