In a 2012 paper, "Gödel's Loophole", F. E. Guerra-Pujol speculates that the problem involves Article V, which describes the process by which the Constitution can be amended. The loophole is that Article V's procedures can be applied to Article V itself. It can therefore be altered in a "downward" direction, making it easier to alter the article again in the future.
This can be practically explored in a logic game called Nomic.