ImageTarget not recognized at all?

So long story short (I hope someone can help me, I feel like I tried everything)

I'm new to Vuforia and Unity AR, so I tried a few (video) tutorials to use Vuforia and create an image target.

I created an Vuforia account and downloaded the latest version for unity (add-vuforia-package-9.8.8.unitypackage). After that I imported that package and created a license key. I added an AR Camera in the unity hierarchy with Rightclick and selecting Vuforia Engine -> AR Camera. I added the license key by selecting "Open Vuforia Engine configuration" and pasting the license key into "App License Key".

I created an ImageTarget by right-clicking in hierarchy again (Vuforia Engine -> Image Target). I set the settings in the inspector and add a simple sphere as child.

Please take a look at the screenshots. They will show the configuration a little clearer. I'm not even using my own image targets yet.

So after all this whenever I try to start it by clicking the Play-button in Unity my webcam turns on and I can see myself. But while holding the printed image targets up to the camera, nothing changes and there is no sphere whatsoever.

I tried without sphere, but instead with Planes or .fbx models. Nothing shows up.

Anyone any idea? I'm really freaking out because everyone else, following the tutorials, seem to have no issues.

PS: I also tried InstantImage after nothing else worked. Still same results.