subdivision creates unwanted creases

Question

tried "remove doubles" on the whole model with bigger threshold with no results, also tried to unmark all edges as sharp/crease. all faces are quads.

link to .blend file

Answer 1

Non-manifold mesh

The issue is caused by subdividing a non-manifold mesh. Here's a pic showing some of the possible situations:

Source: autodesk's maya guide

Your geometry fall in the first case, the "T" shape. With this kind of ambiguous edgeflow, the subdivision surface modifier can brings to unespected results for the user.

You have a bunch of interior faces in coorrispondence with the artifacts:

You should get rid of them to achieve the expected result. As there are just two, I'll suggest to select them manually in edit mode while in wireframe view and delete them (X).