In fact, I had already seen that article, and tried everything in it before posting here. It really is shockingly unhelpful to repeat what a quick search in the support pages returns.
I did remove and recreate the account with the problem, which did not solved nothing. That's what motivated me to try to ask here.
So far, Apple support has been of no use. They won't even admit to the problem. There is little wonder then, that their support articles are of no use to solve this problem. They actually suggested the same thing you did, that it was somehow my service provider that made the configuration change that caused this problem.
Well, I've bounced this off of the support engineers at Dreamhost. They made no changes. In fact, they wouldn't make changes to their PKI infrastructure based on some iOS firmware update that Apple decides to release. Using PKI server certificates to secure communications between clients and servers is an industry standard process which doesn't dance to Apple's tune. Furthermore, hosting services do actually provide services to other than Apple product users. So, no. This has never been the fault of my commercial hosting service.
Apple created this problem when they released iOS 10.2.1. They changed how iOS behaves regarding PKI certificates. Now, if no one here knows exactly what to do to reverse engineer their product to fix a problem they caused, fine. That's how it is.
Did you see the image I posted that clearly shows that it was once possible to choose to trust a certificate? Here's what iOS update 10.2.1 did to that dialog. Note the upper right corner. That is where the option to Trust a certificate once existed:
Do you understand the problem now? This is a problem with how iOS handles SSL certificates, not with the certificates themselves.