You’ll need to join everything together as one mesh (floor and obstacles), then build the navmesh (under scene settings), make sure you have the mesh selected as mentioned above.
[SUP] Ive been using the build navmesh settings which causes the error and when I build one using the physics tab it crashes. I also tried the other ideas yet it still results in error.[/SUP]
Delete the current Navmesh, select the plane, (make sure you are in game engine), choose build navmesh. Used 2.74 and works fine even with just 1 plane.
Nope still not working for me and still saying the same error.
Update: So i updated blender from 2.76 to 2.76b and it seems it solved the problem. Also it seems navmeshes cant generate over largly scale planes about 500 and up.