The rule you refer to translates from the German as:
'A piece cannot be placed if it would cause a square on the board to be surrounded by pieces on three sides'.
This rule must be followed regardless of the colours of the pieces involved.
Thanks for the translation. In two games we tried this interpretation with, we ran into the same problem: If at any time a 4x4 area is enclosed (either by 4 tiles or 3 tiles and the edge of the board), it would create a situation where any tile played in it would violate this rule. A single length tile would cause 3 squares to be surrounded on three sides and a double length tile would cause 2 squares to be surrounded on three sides. Not sure how to deal with that.
We have had the game end early when no more legal placements could be made, but this problem above would have ended the game within the first few turns.