Not sure this is actually artifactory but rather our NGINX reverse proxy.
Basically https://repo.jenkins-ci.org/public/ returns a 405 error code.
This means that Nexus will block the repository as it is an unexpected return code for the root of the repo. It is comon for companies and individuals to run a nexus instance infront of other third party repos.
This has regressed at some point just before the Christmas break and now.