You can attach the model to your post.
I very rarely chose to use thicken, that is a open surface body.
You'd be much better off creating a closed surface quilt (or a planar face at the mirror plane), and then using thicken with the "try to make solid" option. From there you can then use the shell feature to get your wall thickness - and you aren't required to specify a remove face, if you don't want to.
the problem with thicken a surface quite is that it will fail (or give unexpected results) because it's creating additon side walls (3mm) in the process and these are totally dependent on the normal of the surface at that point. Some times the thicken surface will produce self-intersection because of this.