Your TLS configuration is broken, in a way that often happens:
You aren't serving the appropriate intermediates. It's only serving the leaf certificate, but should also serve intermediates that get us to a trusted root.
Because browsers cache intermediates, this obscures the problem much of the time.
This is pretty amazing though. I copied some Go code from Gogland and got it in a browser, syntax highlighting and all! https://ctxt.io/2/AAAAwO9VEA