The fourth exception on Page 3 of the rulebook for the pillbug states:
The Pillbug may not move a piece through a too-narrow gap of stacked pieces (violating the Freedom to Move Rule).
What exactly would be considered a too-narrow gap of stacked pieces?
Our guess was a situation, where the bug to be moved is between two stacks of pieces (two Beetles on top of the hive, both also adjacent to the pillbug). But I don't see how this would violate the Freedom to Move rule.
Another option (ignoring the word stacked): If the bug to be moved would not be able to move by itself (e.g. surrounded by 5 pieces), the pillbug can not move that piece, either.
The English explanation sounds kind of different to the German rule (translated by me):
The Pillbug may not move a piece, if this movement would also move other pieces or if the piece is surrounded.
So, the Pillbug is not able to 'rescue' a surrounded piece? Also, can the Pillbug move a piece into a location (e.g. enclosed space) the piece would not be able to move into?