One of the few useful tools that I have come across which can help you understand why an attempt to create a secure connection fails is the mysterious command tool
nscurl. It’s the tool recommended by Apple for developers who need to do this, although it curiously doesn’t even have a man page. For those who prefer the convenience of a small app to do this rather than having to resort to Terminal, Brolly version 1.0 is now available.
All it does it take the URL that you want to check, call the command
/usr/bin/nscurl --ats-diagnostics --verbose https://www.website.com
where it uses your URL at the end, and displays the result.
This should run on all versions of macOS from El Capitan to Monterey, but I have only been able to test it on Big Sur and Monterey, where it appears to work nicely. It can save the result if you wish, and its Help file and menu contain the link to Apple’s developer page explaining the test in detail, for your convenience.
I’m also looking at additional tests which it can perform, and hope to enhance it in the future if there’s any interest. Why Brolly (British English contraction for umbrella)? It’s all about BROken Links.
Brolly version 1.0 is available from here: brolly1
Should it have any use, I’ll add it to my general software listings. Your comments and suggestions are invited.