When I try to access our dev sites, I get this error on my iPhone X running the latest general release of iOS 15. Now 15.1.
Safari cannot open the page because it could not establish a secure connection to the server.
I believe the problem is related to a root certificate expiring.
On Mac (macOS Big Sur 11.6.1) I was getting the same problem. When I inspected the certificate, it said the R3 and DST Root CA X3 certificates were expired. I was able to fix the issue by installing the R3 intermediate certificate. My Mac already had the ISRG Root X1 certificate installed.
On iOS I've tried installing the root certificates ISRG Root X1 and X2, and the R3 intermediate certificate, and every combination thereof but I still can't load my dev sites.
On Chrome for iOS I get