First of all, thank you for your answer   
Secondly, your code does work - but it's just what I already have. I think even with (or because?) your Dr. Pepper and Epiphany you misunderstood me.
The problem I'm stuck to is: A Sprite on Layer 2 can collide with a Tile on Layer 1. That wouldn't be a problem yet. Now since the whole map is parallax-scrolling, I have the problem that the coordinates (50,50) on Layer 1 aren't at the same place as the coordinates (50,50) on Layer 2, so I have to calculate which pixels on Layer 1 are just below/in front of the Sprite on Layer 2. The whole thing is kinda 3D. Well, almost.

I'm working on a "simple" game engine of which the display is already working.
It works as follows:

• A map can consist of multiple layers
• Each layer can contain sprites and tiles
• Sprites and tiles both can be either solid or non-solid
• Sprites and tiles both can be multiple layers "tall"
• the top-left corner of all layers is at 0,0 and the parallax effect is achieved using a different scroll factor for each layer.

Assumed I have a game consisting of two layers.
Layer 1 has a scroll factor of 0.5 and contains a solid tile which is 2 layers tall (layer 1 and 2)
Layer 2 has a scroll factor of 1 and contains a solid sprite which is 1 layer tall (only layer 2)
How do I compute if the sprite collides with the tile on the "projection layer" as seen by the player (in this case it's layer 2 since it has a scroll factor of 1)?