NavMeshagent.hasPath==false despite obvious path

Guys,

I got a couple of models using Navmesh to do their patrolling. That works fine on my simple and straight test surfaces. As soon as I go in my level - which has rooms and hallways things get strange.
Now the Navmeshagent complains NavMeshagent.hasPath==false, while isOnNavmesh is true. If I debug draw a line from current position to destination I can see that it is obviously on the mesh.

I am using NavMeshSurface so I can disable and enable the mesh. If I do this, 1, 2 or three times the agent will eventually set hasPath to true, walk to the position, following the obvious path. That will work for 1, 2 or 3 waypoints than it will stop again.
I can repeat this cycle.

I have rebaked the mesh a dozen times, but the mesh is obviously not the problem.

Now I am out of ideas, anybody having any ideas what to try?

Thanks a lot!