I had a computer expert chasing all caches in my laptop (including any hidden ones), clean registries and cookies, but to no avail. Everything was pointing to something else, external to my laptop.
So, with no other options left, I reached out to our department expert. He was kind to look into it, and after investigating, he explained me what he thinks the problem is. Because I am not computer literate, I asked him to write it to me. I hope his message makes sense for you and offers the explanation:
Roy,
Using the SSLLabs website, I first checked to see what the certificate(s) were on the website:
View attachment 190656
Here it shows 2. One is on an IPv4 Address (bottom) and the other is on an IPv6 Address (top). The IPv4 is good, and the other has an issue with the DNS name mismatching the certificate CN.
I then took a look at the DNS resolution from my laptop. Here you can see the IPv4 (45.33.78.185) and IPv6 (2600:3c03::f03c:94ff:feb6:f300) addresses that match the servers. I used Google’s DNS Servers (8.8.8.8) in this example but got consistent reports from our internal / default server.
View attachment 190657
I also saw the same behavior as you with our work network resolving to the IPv4 and the good certificate and a mobile carrier resolving IPv6 and a bad certificate. I believe the difference is the default use of an IPv4 vs IPv6 address on the different networks. Both should work, but it appears that there is a misconfiguration on the IPv6 side of the website. I found a thread of a similar issue linked below:
IPv6 Certificate not valid for domain name , IPv4 is correct
Thanks