It does find the

task.dokkaHtml

one. Any ideas how I can solve this? Thx.

Could you please share your buildscript or create an MRE for that?

Sure, here it is:

main

buildscript {
    repositories {
        google()
        jcenter()

    }
    dependencies {
        classpath(kotlin("gradle-plugin", Version.kotlinVersion))
        classpath("com.android.tools.build:gradle:4.0.2")
        classpath("org.jetbrains.dokka:dokka-gradle-plugin:${Version.dokka}")
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

tasks.register("clean").configure {
    delete("build")
}

tasks.dokkaHtmlMultiModule.configure {
 outputDirectory.set(buildDir.resolve("dokkaCustomMultiModuleOutput"))
}

submodule

plugins {
    id("com.android.library")
    id("github-publish")
    kotlin("android")
    id("org.jetbrains.dokka")
}
....

dependencies {

    implementation(kotlin("stdlib", Version.kotlinVersion))

    api(Lib.KotlinX.coroutinesCore)
    api(Lib.KotlinX.coroutinesAndroid)

    api(Lib.mqtt)

    testImplementation(Lib.Test.junit4)
    testImplementation(Lib.Test.junit5Api)
}

tasks.dokkaHtml.configure {
    outputDirectory.set(buildDir.resolve("dokka"))
}