Solving "mbedtls_ssl_handshake failed: -30848" - where to start?

I’m building functionality in the Pico_w. I’ve got TLS working and can connect to sites like openweathermap
on port 443. However, other sites don’t work. The example is
In a browser I use “” but from the Pico I’m getting the message in the title.

I’ve browsed similar questions and all of the relevant DEFINE’s suggested in those seem to be set in the Pico implementation

As per the question, how do I start to diagnose this? I am very much NOT an encryption expert or even a beginner