Someone sent me a .stl files that Cura doesn't like. Here is an easy way to fix the file.

• Go to https://stephaneginier.com/sculptgl/
• Scene > Clear scene
• File > add (upload your .stl)
• In the right menu bar, under Voxel Remeshing, push it to 400 (the bigger the number, the better the quality, the heavier the file).
• Click “Remesh”
• File > Save .stl
• Done! Now Cura likes the new file.