Freecad has a famously bad boolean solver, but creating temporary booleans is essential to split models up for printing.  When it fails, it normally leaves off a face.  Cura can't slice it unless it's a closed solid.  

Cura shows a goofy pattern where a boolean error has happened & it slices it into garbage.  Fortunately, you have a friend in the boolean business: Blender.  Just go to file->import to import the STL file.

Blender shows 2 faces are missing.  It's just a matter of going into edit mode, hitting C to select the vertices around the holes & hitting F to create faces.

Then, export it as an STL file.  Cura is happy to load the STL file generated from blender, with all the right dimensions.

Booleans in Freecad are a pain in the mane anyways, with goofy offsets & goofy tweeks normally required.  Blender can import the STL files for the model & print mask.

Then apply the boolean by "adding a modifier" to the object to be printed & hiding the print mask object.  Of course, we still need a tiny offset to get rid of some dangling faces.  At least blender shows the dangling faces instead of completely failing.

Blender converts 1mm to 1 meter.  .1 meter in Blender is .1mm.

You now have to export the STL file with "apply modifiers" enabled.

Cura slices booleans from Blender just as well as Freecad.