This message occurs when the Cgeometry_Velocity render element is present and enabled, and the scene contains meshes with animated changing topology.
The Cgeometry_Velocity render element will be rendered only for objects without a changing topology.
Possible causes:
Presence of mesh(es) with animated changing topology in scene with an active Cgeometry_Velocity render element.
Solutions:
Unfortunately there is no known solution to reliably compute topological motion blur on meshes with changing topology, unless motion vector data were computed in advance and stored within mesh vertices.