It seems the plugin portal has gone havoc.
Try opening https://plugins.gradle.org/m2/com/github/johnrengelman/shadow/com.github.johnrengelman.shadow.gradle.plugin/2.0.4/com.github.johnrengelman.shadow.gradle.plugin-2.0.4.jar, it will not load.
That URL should not load. There should not be a JAR at the coordinates of the plugin marker interface.
The actual artifact is https://plugins.gradle.org/m2/com/github/jengelman/gradle/plugins/shadow/2.0.4/shadow-2.0.4.jar
as indicated in
https://plugins.gradle.org/m2/com/github/johnrengelman/shadow/com.github.johnrengelman.shadow.gradle.plugin/2.0.4/com.github.johnrengelman.shadow.gradle.plugin-2.0.4.pom
Current behavior is that this JAR is checked when resolving plugins, but redirects to JCenter. If JCenter doesn’t return a 404 quickly, the error is misleading. There’s been much discussion about this and the resulting issue is https://github.com/gradle/gradle/issues/6155.
Ah, so it is a laggy JCenter that causes the build to fail with “HEAD request not possible”? I see.
Is there any estimate when this is worked around in the portal?