Instrumentation Tests faild due to NoClassDefFoundError

I'm multidexing my kotlin app since today in the debug mode because a library has forced me to do so. My problem is that my UI-Tests aren't working anymore. I'm also minifying my apk.

I'm getting the following error:

FATAL EXCEPTION: MockWebServer
Process: [package], PID: 19446
java.lang.NoClassDefFoundError: Failed resolution of: Lkotlin/collections/CollectionsKt;
    at [package].activity.MockedServerTestsBase$dispatcher$1.dispatch(Unknown Source)
    at okhttp3.mockwebserver.MockWebServer$3.processOneRequest(Unknown Source)
    at okhttp3.mockwebserver.MockWebServer$3.processConnection(Unknown Source)
    at okhttp3.mockwebserver.MockWebServer$3.execute(Unknown Source)
    at okhttp3.internal.NamedRunnable.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
    at java.lang.Thread.run(Thread.java:761)
Caused by: java.lang.ClassNotFoundException: Didn't find class "kotlin.collections.CollectionsKt" on path: DexPathList[[zip file "/system/framework/android.test.runner.jar", zip file "/data/app/[package].test-2/base.apk", zip file "/data/app/[package]-1/base.apk"],nativeLibraryDirectories=[/data/app/[package].test-2/lib/x86, /data/app/[package]-1/lib/x86, /system/lib, /vendor/lib]]
    at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:380)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
    at [package].activity.MockedServerTestsBase$dispatcher$1.dispatch(Unknown Source) 
    at okhttp3.mockwebserver.MockWebServer$3.processOneRequest(Unknown Source) 
    at okhttp3.mockwebserver.MockWebServer$3.processConnection(Unknown Source) 
    at okhttp3.mockwebserver.MockWebServer$3.execute(Unknown Source) 
    at okhttp3.internal.NamedRunnable.run(Unknown Source) 
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133) 
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607) 
    at java.lang.Thread.run(Thread.java:761) 

Has anyone received this error who is also using the OkHttp MockWebServer? Or something else. Maybe I'm missing some proguard rules?

My app/build.gradle file:

android {
    defaultConfig {
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        debug {    
            debuggable true
            minifyEnabled true
            dexOptions {
                multiDexEnabled true
            }
    }
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib:1.1.60"

    androidTestImplementation "com.squareup.okhttp3:mockwebserver:3.8.1"
    androidTestImplementation "com.android.support.test:runner:1.0.1"
    androidTestImplementation "com.android.support.test:rules:1.0.1"
}

I'd be thankful for any help and/or discussion if my approach is right.