Failure to resolve some plugins from certain repos

Trying to test a Jenkins build with a new plugin, by adding this line with the rest of the test plugins in my build.gradle file:

testPlugins 'org.jenkins-ci.plugins:parameter-separator:1.3'

but seeing a FATAL error when downloading the POM file like this:

Starting a Gradle Daemon, 1 incompatible and 1 stopped Daemons could not be reused, use --status for details
:clean UP-TO-DATE
:compileJava NO-SOURCE
:compileGroovy
Download https://repo1.maven.org/maven2/org/codehaus/groovy/groovy-all/2.4.11/groovy-all-2.4.11.pom
Download https://repo.jenkins-ci.org/releases/org/jenkins-ci/plugins/job-dsl-core/1.70/job-dsl-core-1.70.pom
[Fatal Error] job-dsl-core-1.70.pom:7:3: The element type "hr" must be terminated by the matching end-tag "</hr>".
Download https://repo1.maven.org/maven2/org/kohsuke/github-api/1.93/github-api-1.93.pom
Download https://repo1.maven.org/maven2/org/kohsuke/pom/17/pom-17.pom
Download https://repo1.maven.org/maven2/org/apache/commons/commons-lang3/3.7/commons-lang3-3.7.pom

From what I can tell, anything served from the “repo.jenkins-ci.org” path serves some HTML landing page with a frog icon instead of just serving the POM file right away, and that’s causing it to fail parsing the POM file with errors like:

* What went wrong:
Could not resolve all dependencies for configuration ':compileClasspath'.
> Could not resolve org.jenkins-ci.plugins:job-dsl-core:1.70.
  Required by:
      project :
   > Could not resolve org.jenkins-ci.plugins:job-dsl-core:1.70.
      > Could not parse POM http://repo.jenkins-ci.org/releases/org/jenkins-ci/plugins/job-dsl-core/1.70/job-dsl-core-1.70.pom
         > The element type "hr" must be terminated by the matching end-tag "</hr>".

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

Hmm, seems that the maven url was missing the s in https, and that was causing all these problems.

The lines in the build.gradle file should have looked like this, so the URL uses https instead of http.

repositories {
    mavenCentral()
    maven { url 'https://repo.jenkins-ci.org/releases/' }
    jcenter()
}