I find myself unable to diagnose the problem here.
Both curves are planar at position z 0. “Intersect” finds an intersecting point. Yet running “split” results in an error. I’m getting this a lot in another file I am working on too. Just can’t find a reason…
I don’t think you can split a closed curve at a single point with Split. The resulting error message is misleading/incorrect because the objects are within tolerance. Add another line crossing the circle, run Split, select both lines as the splitting objects and the circle is split into two parts.