Thanks for the detailed update.
I’m officially out of ideas, seeing as: everything works for me on that same macOS version, and: someone who used to have the x509 error with previous butler versions no longer has it since the update.
My only idea would be to find the corresponding Root certificate and see if the trust settings are set to “Always trust” or “Custom”, see https://github.com/golang/go/issues/24652#issuecomment-378340252