Fork does not recognize my instrumentation APK

[GXGOW]

So I've been trying to get Fork up and running for some time now for a research project, but I keep stumbling upon the same error:

12:48:19.963 [DEBUG] [org.gradle.process.internal.DefaultExecHandle] Changing state to: STARTING
12:48:19.963 [DEBUG] [org.gradle.process.internal.DefaultExecHandle] Waiting until process started: command '/usr/lib/jvm/java-8-openjdk/bin/java'.
12:48:19.964 [DEBUG] [org.gradle.process.internal.DefaultExecHandle] Changing state to: STARTED
12:48:19.964 [INFO] [org.gradle.process.internal.DefaultExecHandle] Successfully started process 'command '/usr/lib/jvm/java-8-openjdk/bin/java''
12:48:19.964 [DEBUG] [org.gradle.process.internal.ExecHandleRunner] waiting until streams are handled...
12:48:21.028 [DEBUG] [org.gradle.launcher.daemon.server.Daemon] DaemonExpirationPeriodicCheck running
12:48:21.029 [DEBUG] [org.gradle.cache.internal.DefaultFileLockManager] Waiting to acquire shared lock on daemon addresses registry.
12:48:21.029 [DEBUG] [org.gradle.cache.internal.DefaultFileLockManager] Lock acquired on daemon addresses registry.
12:48:21.029 [DEBUG] [org.gradle.cache.internal.DefaultFileLockManager] Releasing lock on daemon addresses registry.
12:48:21.029 [DEBUG] [org.gradle.cache.internal.DefaultFileLockManager] Waiting to acquire shared lock on daemon addresses registry.
12:48:21.029 [DEBUG] [org.gradle.cache.internal.DefaultFileLockManager] Lock acquired on daemon addresses registry.
12:48:21.029 [DEBUG] [org.gradle.cache.internal.DefaultFileLockManager] Releasing lock on daemon addresses registry.
12:48:21.430 [QUIET] [system.out] INFO  TestClassMatcherInjector:31 - Fork will try to find tests in org.hogent.nicolas.loots.scalabletestingconcept from your instrumentation APK.
<devices found>
12:48:23.596 [QUIET] [system.out] ERROR ForkRunner:81 - Error when trying to find test classes
12:48:23.596 [QUIET] [system.out] com.shazam.fork.suite.NoTestCasesFoundException: No tests cases were found in the test APK: /mnt/DATA/Users/nicol/stack/3TIN/Bachelorproef/bachproef/fork/apk/androidTest/debug/app-debug-androidTest.apk
12:48:23.596 [QUIET] [system.out] 	at com.shazam.fork.suite.TestSuiteLoader.loadTestSuite(TestSuiteLoader.java:57)
12:48:23.596 [QUIET] [system.out] 	at com.shazam.fork.ForkRunner.run(ForkRunner.java:62)
12:48:23.596 [QUIET] [system.out] 	at com.shazam.fork.Fork.run(Fork.java:46)
12:48:23.596 [QUIET] [system.out] 	at com.shazam.fork.ForkCli.main(ForkCli.java:106)

I know that the instrumentation APK is working, because I can deploy it without issues through the command line task, or with frameworks like Spoon or Composer. I've tried running fork through both the standalone version and the gradle plugin, but both return the same error. I'm kind of stuck, so could anyone please help me getting this up and running!

This is my config.json (for the standalone version):

{
	"title" : "Fork Report",
	"subtitle" : "automatically split phones to tablets",
	"poolingStrategy" : {
		"eachDevice": true
	}
}

I run fork with the following command:
./gradlew fork-runner:run --debug --stacktrace -Pargs=' --apk apk/debug/app-debug.apk --test-apk apk/androidTest/debug/app-debug-androidTest.apk --config config.json'

The given paths do exist, so that shouldn't be the issue I think. I'm running these commands on an Arch Linux machine (Linux kernel 4.16).