It's a Workplane problem.
I suspect you were in 3D mode when you selected the polyline and performed the Mirror copy. Your mirror copy is on a different workplane (flipped over). If you select the errant polyline first (to switch to its WP), then you should be able to use the Fillet2D tool without a problem. Otherwise, select the bad polyline, activate "Plane by World" and "Place on Workplane", then Fillet2D.
« Last Edit: March 18, 2018, 02:24:17 PM by John R »
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