Thanks, Matt! I just saw, that there's no camera assigned to the image map shader in the file I attached. I must have forgotten it. Embarrassing, since I know that!
I tested my file with a camera assigned, and the bucket artifacts were gone.
But at the moment I'm rendering my original scene, and the camera
is assigned to the shader (see image below). The problem is still there.
I even copied the card object from the scene I attached here (which looked correct after I assigned the camera) and pasted it into my original scene insted of the imported plane, and the artifacts are still there! So, could this be a memory issue? There are many object populations and detailed objects in the scene, but I have 32 GB of RAM. Weird...