Strange deformation after adding subdivide surface

So when I add the subdivide surface modifier to the plane wich has also a mirror modifier on it. (None of them are applied)
This happens.
[ATTACH=CONFIG]370249[/ATTACH]
If it’s not clearly visible.
[ATTACH]370250[/ATTACH]
That’s the Blender file.

Turn off the subsurf and actually look at the base mesh

Ngons - faces with more than four sides = BAD


Edges that are not part of the underlying faces. Delete those edges and instead connect verteices with shortcut J or K for the knife tool (or delete the faces and refill the holes)


That’s actually what I had done before posting.
Deleted again, filled it up again.
Works like a charm, thx for the advice!