Module dependency leads to Error:Gradle: Execution failed for task ':app:preDexDebug' in Android Studio

I tried to add a module to an Android project and make the main module dependent on it and I got this error:

Error:Gradle: Execution failed for task ':app:preDexDebug'. com.android.ide.common.process.ProcessException: org.gradle.process.internal.ExecException: Process 'command 'C:\Program Files\Java\jdk1.8.0_40\bin\java.exe'' finished with non-zero exit value 1

I have a project with the main module app and added a module mylib and app depends on mylib.

Android Studio 1.2.2.

settings.gradle:

include ':app', ':mylib'

build.gradle from app:

    apply plugin: 'com.android.application'

android {
    compileSdkVersion 22
    buildToolsVersion "21.1.2"

    compileOptions {
        sourceCompatibility org.gradle.api.JavaVersion.VERSION_1_8
        targetCompatibility org.gradle.api.JavaVersion.VERSION_1_8
    }

...

}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    compile project(':mylib')
}

build.gradle from mylib:

apply plugin: 'java'

sourceCompatibility = org.gradle.api.JavaVersion.VERSION_1_8
targetCompatibility = org.gradle.api.JavaVersion.VERSION_1_8

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
}

The JAVA_HOME variable:

JAVA_HOME=C:\Program Files\Java\jdk1.8.0_40

The problem starts when I add line compile project(':mylib') to build.gradle, sometimes when I rebuild the project, but always when I try to run it. The VERSION_1_8 statements don't solve the problem.

What can be wrong? Any help will be very much appreciated.