Ich entwickle eine Android-Anwendung mit RxJava und Retofit. Wenn ich meinen Code auf API 23 teste, erhalte ich keine Fehler, aber wenn ich in API 15 teste, scheint mein RxJava nicht richtig zu funktionieren und meine App stürzt mit folgendem Fehler ab:

FATAL EXCEPTION: main
java.lang.ExceptionInInitializerError
at $Proxy1.createToken(Native Method)
at com.loginsignup.presenter.LoginPresenterImpl.tryToLogin(LoginPresenterImpl.java:39)
at com.loginsignup.view.LoginActivity.tryToLogin(LoginActivity.java:197)
at com.loginsignup.view.LoginActivity.onClick(LoginActivity.java:180)
at android.view.View.performClick(View.java:4084)
at android.view.View$PerformClick.run(View.java:16966)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NoClassDefFoundError: rx.plugins.RxJavaPlugins
at rx.Observable.<clinit>(Observable.java:63)
at $Proxy1.createToken(Native Method) 
at com.gooroo.gooroo.loginsignup.presenter.LoginPresenterImpl.tryToLogin(LoginPresenterImpl.java:39) 
at com.gooroo.gooroo.loginsignup.view.LoginActivity.tryToLogin(LoginActivity.java:197) 
at com.gooroo.gooroo.loginsignup.view.LoginActivity.onClick(LoginActivity.java:180) 
at android.view.View.performClick(View.java:4084) 
at android.view.View$PerformClick.run(View.java:16966) 
at android.os.Handler.handleCallback(Handler.java:615) 
at android.os.Handler.dispatchMessage(Handler.java:92) 
at android.os.Looper.loop(Looper.java:137) 
at android.app.ActivityThread.main(ActivityThread.,java:4745) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:511) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
at dalvik.system.NativeStart.main(Native Method)

Hier ist mein gradle Build:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.2"

    defaultConfig {
        applicationId "com.example.ex"
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "0.1 alpha"
        multiDexEnabled true
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    dexOptions {
        javaMaxHeapSize "4g"
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    testCompile 'junit:junit:4.12'
    debugCompile 'com.squareup.leakcanary:leakcanary-android:1.4-beta2'
    releaseCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.4-beta2'
    testCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.4-beta2'
    compile files('libs/sinch-android-rtc-3.9.5.jar')
    compile 'com.android.support:appcompat-v7:23.4.0'
    compile 'com.android.support:design:23.4.0'
    compile 'com.android.support:support-v4:23.4.0'
    compile 'com.android.support:cardview-v7:23.4.0'
    compile 'com.google.android.gms:play-services-maps:8.4.0'
    compile 'com.google.android.gms:play-services-location:8.4.0'
    compile 'com.google.code.gson:gson:2.6.2'
    compile 'com.google.android.gms:play-services-analytics:8.4.0'
    compile 'com.squareup.picasso:picasso:2.5.2'
    compile 'com.squareup.okhttp3:okhttp-urlconnection:3.2.0'
    compile 'com.squareup.okhttp3:okhttp:3.2.0'
    compile 'com.squareup.okhttp3:logging-interceptor:3.2.0'
    compile 'com.squareup.retrofit2:retrofit:2.0.0'
    compile 'com.squareup.retrofit2:converter-gson:2.0.0'
    compile 'com.squareup.retrofit2:adapter-rxjava:2.0.0'
    compile 'com.github.orhanobut:logger:1.12'
    compile 'com.viewpagerindicator:library:2.4.1@aar'
    compile 'io.reactivex:rxandroid:1.2.0'
    compile 'io.reactivex:rxjava:1.1.5'
    compile 'com.facebook.android:facebook-android-sdk:4.+'
    compile 'com.stripe:stripe-android:+'
    compile 'com.google.android.gms:play-services-gcm:8.4.0'
    compile 'com.timehop.stickyheadersrecyclerview:library:0.4.3@aar'
    compile 'com.amazonaws:aws-android-sdk-core:2.+'
    compile 'com.amazonaws:aws-android-sdk-s3:2.+'
    compile 'com.google.android.gms:play-services-wallet:8.4.0'
    compile 'com.android.support:multidex:1.0.0'
}

apply plugin: 'com.google.gms.google-services'

Keine Ahnung, warum es in API 23 kein Problem gibt, aber wenn ich es in API 16 ausführe, stürzt es ab, wenn ich versuche, mich anzumelden, bei dem RxJava verwendet wird?