Dear @tobias
the edge in question is a 180 or 360-degree edge. Quite sure this will generate special cases, as for example the cross product will generate a 0 (vector).
solutions / workarrounds:
(1)
to get a fast solution
_offsetSrf -10 (to the inside)
_offsetSrf 10 with corner = round (to outside)
(2)
prepare the edge to allow filleting:
_extractSrf (the radial surface, copy = no)
_split (isocurve, shring=yes) a quite small part near the edge
_extendSrf (line, merge=yes)
_createSolid (or trim,join …)
→ filletEdge R10 works fine
see attached file
Fillet-2_tp.3dm (510.2 KB)