Based on the excerpt provided, the one potential problem with the Articles of Confederation seems to be related to the voting power of states in Congress. Specifically, the statement that "smaller states had the same voting power in Congress as larger states" reflects a significant issue because it creates a disproportionate influence of smaller states compared to their population size. This imbalance was a concern that ultimately contributed to the drafting of the U.S. Constitution.
While the other issues mentioned (such as the lack of civilian control of the military, Congress having the power to declare war, and states being unable to deal directly with foreign countries) are notable, the equal voting power of states regardless of size is often highlighted as a significant weakness in the Articles of Confederation leading to calls for reform.
Therefore, the best answer to your question is: smaller states had the same voting power in Congress as larger states.