Let $n,k \in \mathbb{N}-\{0,1\}$. The generalization of the closed knight's tour problem to higher dimensions asks to move a knight along the $n^k$ cells of a $n \times n \times \cdots \times n$ checkboard belonging to the Euclidean space $\mathbb{R}^k$, touching the centres of all of them only once, and then returning to the starting square at move $n^k$.
Now, as suggested by the definition provided by the FIDE LAWS of CHESS (see Article 3.6), assume that the knight is a piece whose move rule states that it can move to any square/cell of the chessboard that is $\sqrt{5}$ (chessboard) units away from the square where it stands.
Then, assume $n=2$. I have proven here that such a closed knight's tour is always possible as long as $k>6$, while it cannot clearly exists any (possibly open) knight's tour for any $k<6$ (i.e., a knight can perform its special move only once on the $2 \times 2 \times 2 \times 2 \times 2$ board, from a random vertex of the hypercube to the opposite vertex and then there are no other — unvisited — vertices at a distance greater than or equal to $\sqrt{5}$ from the current one).
My question is to find an easy proof that no closed knight's tour exist on the $2 \times 2 \times 2 \times 2 \times 2 \times 2$ board.