Hi
As you indicate a number of nodes (95728), it's possible that matisse generated a wrong mesh (it's a rare situation, except if you modified the generated mesh (move of a node, exchange of a segment).
Try to use the menu "check the mesh"...
If Matisse says correct mesh then it could be one of the reason Jean-Michel wrote.
If you have an incorrect mesh, Matisse should highlight the node location (where the problem is) and you could try to see why the mesh is wrong.
Good luck
Christophe