Why are my lowest layers not solid?
I printed the thing for some layers to see what the printer actually does:


There are clearly too many retractions and unretractions.

This can be seen in advance by letting Slic3r show the retractions and unretractions. 




For the green "walls", it became clear that it prints them exactly onto the empty space, so in fact the seem to be floating. 

That made it obvious to me: the normals were pointing into the wrong direction. Therefore, Slic3r seems to have interpreted the material side of the green parts inside out. I had the same issue on some other parts as well.

I went to Blender, selected the object(s), chose edit mode and found a function called "Recalculate normals". Since then, the bottom layer is solid.

Also printing time is reduced by half an hour.