Solidify Modifier changes thickness unintentional

Hi there,

the Solidify Mod is causing problems for me. I have modeled a plane with all vertices at z=0 and gave it a solidify modifier with a small thickness. Now if I rotate a part of my model (so that some vertices move on z), the thickness changes all of a sudden and gets extremely thick.

before with thickness = 1mm:


after with thickness = obviously not 1mm:


I need everything stay with the thickness at exact 1mm.

What and why is this happening?