I first got the ERR_SSL_PROTOCOL_ERROR from my front end web service when it tries to connect to my back end web service using its HTTPS URL.
If I then try and use the same URL to my back end web service in Chrome, it fails with a ERR_SSL_PROTOCOL_ERROR.
Chrome has an info on the left of the URL which when I click on it says “Your connection to this site is not secure…”
Could this be some Render proxy configuration problem?
Could it be related to Chrome using strict-origin-when-cross-origin and the way the Render proxy handles SSL?
Just FYI…
I moved the backend service to Google Cloud Run and it connects fine over HTTPS, so it does appear to be something wrong with the Render proxy / HTTPS configuration.
As Cloud Run works for the foreground web server as well, I have moved both to Cloud Run, as it just worked.