Hey,
sorry to dig up this old thread, but I found it via the search and thers that may find this too should maybe know this:
To make this work with tiled backgrounds, you have to use a block-element (no sprite) with shadow-caster behaviour and properties "solid", and the sprite or tiled background on top of that (work with layers here, because you aren't allowed to make the shadow-caster block just beneath the tiled-background on the same layer - it leads to strange results). Making the shadow-caster block invisible at runtime via events also don't work, the shadow-caster element will need to be visible (although beneath another sprite/tiled background) to cast a shadow.
I used different blocks with standardized heights (50 for walls, 10 for high objects, 1-2 for obstacles like crates etc).