Hi Rogurt,

The problem is the two last surfaces as shown in this picture.



If you zoom up on one of these surfaces you'll see the ends have a tight bend in them as shown below,
if the radius of the bend is tighter than the shell thickness then it won't work, if you apply a thickness of 0.2 it will thicken,
but if you zoom in on that area you'll see it starting to look funky, I did notice that you can apply a thickness of 1 with the
'Direction' option to 'Flip' but again you get some funky surfaces happening. If you try thickening excluding those two surfaces it works fine,
so if you tidy up the two end surfaces to have less of a tight bend it should work out.



Cheers