The problem appears to be an incompatibility with this boiler-plate script I have in

~/.gradle/init.d/scans.init.gradle.kts

to enable Gradle build scans in all my builds, with the following content:

import com.gradle.enterprise.gradleplugin.GradleEnterprisePlugin
import com.gradle.scan.plugin.BuildScanPlugin
import org.gradle.util.GradleVersion

initscript {
  val pluginVersion = "3.8.1"

  repositories {
    gradlePluginPortal()
  }
  dependencies {
    classpath("com.gradle:gradle-enterprise-gradle-plugin:${pluginVersion}")
  }
}

val isTopLevelBuild = gradle.getParent() == null
if (isTopLevelBuild) {
  val gradleVersion = GradleVersion.current().baseVersion
  val atLeastGradle5 = gradleVersion >= GradleVersion.version("5.0")
  val atLeastGradle6 = gradleVersion >= GradleVersion.version("6.0")

  if (atLeastGradle6) {
    settingsEvaluated {
      if (!pluginManager.hasPlugin("com.gradle.enterprise")) {
        pluginManager.apply(GradleEnterprisePlugin::class)
      }
      extensions["gradleEnterprise"].withGroovyBuilder {
        configureExtension(getProperty("buildScan"))
      }
    }
  } else if (atLeastGradle5) {
    rootProject {
      pluginManager.apply(BuildScanPlugin::class)
      configureExtension(extensions["buildScan"])
    }
  }
}

fun configureExtension(extension: Any) {
  extension.withGroovyBuilder {
    setProperty("termsOfServiceUrl", "<https://gradle.com/terms-of-service>")
    setProperty("termsOfServiceAgree", "yes")
    // other configuration
  }
}

This works fine with 1.6.10.