Hi Nikita,

re:
> When I filleted the bottom surface too, and then applied the shell construct I have no problems.
> Why is that?

Offsets are easier to calculate when the pieces meet smoothly.

An offset between 2 smooth pieces goes something like this:





With smoothly meeting pieces, the offsets of each piece automatically touch each other.

With pieces that meet at a sharp corner that isn't the case:





With sharp corners, the offsets of each piece do not automatically touch each other, a gap opens up between them. So there has to be a process of extending and intersecting the pieces which is especially difficult when it's surfaces involved instead of curves.

- Michael