Spotless gradle plugin not working on closed system

I am using gradle 5.4.1. I am using spotless-plugin-gradle 3.25.0.

My build.gradle contains

plugins {
  id 'java'
  id 'eclipse'
  id 'maven-publish'
  id 'com.github.johnrengelman.plugin-shadow' version '2.0.3'
  id 'org.sonarqube' version '2.7.1'
  id 'com.diffplug.gradle.spotless' version '3.25.0'
}

I have spotless in nexus on my closed system (it cannot reach the internet). The gav for spotless in my nexus repository is 'com.diffplug.spotless:spotless-plugin-gradle:3.25.0'.

My settings.gradle file contains:

pluginManagement {
  resolutionStrategy {
    eachPlugin {
      if (requested.id.namespace == "org" && requested.id.name == "sonarqube") {
        useModule("org.sonarsource.scanner.gradle:sonarqube-gradle-plugin:${requested.version})
      }
      if (requested.id.namespace == "com.github.johnrengelman" && requested.id.name == "plugin-shadow") {
        useModule("com.github.johnrengelman.gradle.plugins:shadow:${requested.version})
      }
      if (requested.id.namespace == "com.diffplug.gradle" && requested.id.name == "spotless") {
        useModule("com.diffplug.spotless:spotless-plugin-gradle:${requested.version})
      }
    }
  }
  repositories {
    maven {
      url "http://nexus:8081/repository/jcenter"
    }
  }
}

When I do a "./gradlew dependencies" I get the following error:

An exception occurred applying plugin request [id: 'com.diffplug.gradle.spotless' version: '3.25.0', artifact: 'com.diffplug.spotless:spotless-plugin-gradle:3.25.0']
> Failed to apply plugin [id: 'com.diffplug.gradle.spotless']
 > Could not create an instance of type com.diffplug.gradle.spotless.SpotlessExtension
  > Could not generate a decorated class for class com.diffplug.gradle.spotless.SpotlessExtension
   > com/diffplug/spotless/LineEnding

the stacktrace didn't seem to help much. I tried the old approach (buildscript block) but that results in the same error. Can someone point out what I am doing wrong? On a system where I have direct access to the internet (and thus don't need to add any entries to the settings.gradle file) everything works just fine.

Bjorn Vester hit the nail on the head:

It looks like a missing class definition to me. Did you upload all the plugin's dependencies and its original pom file?