Even if I draw only two strokes, the same error occurs.
I did exactly what you’re doing (added Suzanne with three subdivisions, added a cube, deleted the cube, drew strokes in Edit Mode, checked Keep Strokes) and I still get the same error.
Solved the issue. It was in the fact that I had BSurfaces installed in two places on my computer. Maybe it’s bundled with Blender and I didn’t realize it, so I installed another one on top of that, causing the issue. I deleted the one I installed and now it works.