Have there been recent changes in the package mirror options?
I have some servers behind a firewall with a proxy for outbound traffic. I've successfully used the Revolution Analytics mirror for packages for a couple of years, but it stopped working a few weeks ago. I had not changed anything on the proxy whitelist or in R or Rstudio on the servers. I had the "Use secure download method" box checked, but I tried it checked and unchecked. I have the latest RStudio on all servers, with a mix for now of 3.6.0 and 3.6.3 R versions.
After some troubleshooting, I tried the main CRAN mirror and got it to work, I guess because it's http. Is there another setting I need to change to get an https mirror to work?