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.
If it’s not clearly visible.
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!