I have the following configuration for a Springboot application:

DemoApplication.kt

package com.planes.planesserver

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class DemoApplication

fun main(args: Array<String>) {
     runApplication<DemoApplication>(*args)
}

build.gradle.kts

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile


plugins {
    id("war")
    id("org.springframework.boot") version "3.2.0"
    id("io.spring.dependency-management") version "1.1.4"
    kotlin("jvm") version "1.9.21"
    kotlin("plugin.spring") version "1.9.20"
    kotlin("plugin.jpa") version "1.9.20"
    id("org.flywaydb.flyway") version "7.15.0"
}

group = "com.planes.planesserver"
version = "0.0.1-SNAPSHOT"
java {
     sourceCompatibility = JavaVersion.VERSION_17
}

repositories {
    mavenCentral()
}

dependencies {
//here list of dependencies
}

tasks.withType<Test> {
    useJUnitPlatform()
}

tasks.withType<Jar> {
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE

    manifest {
         attributes["Main-Class"] = "com.planes.planesserver.DemoApplication"
    }
}


tasks.withType<KotlinCompile> {
    kotlinOptions {
        freeCompilerArgs = listOf("-Xjsr305=strict")
        jvmTarget = "17"
    }
}

springBoot {
    mainClass.set( "com.planes.planesserver.DemoApplication")
}

//other stuff

I am deploying with the bootJar gradle task. When trying to run the jar with java -jar I get the error: main class cannot be found or loaded.
I unpacked the jar archive. Under BOOT-INF/classes/com/planes/planesserver there exists the file DemoApplication.class as well as the DemoApplicationKt.class

Can anyone give me a hint in the right direction ?