English Translation
Her explanation of the problem was nonsense.
無意味 in this sentence doesn’t function as “nonsense” but “ineffective” or “not fruitful”. 彼女の説明は無意味だった is more likely to be interpreted as this way: Her explanation was actually insightful and made a lot of sense, but it was just too late to change the decision made by her boss.
The English sentence can be translated in Japanese as:
その問題に対する彼女の説明は的外れだった。（まとはずれ）
その問題に対する彼女の説明は意味不明だった。（いみふめい）
的外れ consists of 的（target）and 外れる（to be diverted）. It’s like an amateur archery game. 的外れな as an adjective is used when someone says completely irrational, or off-topic.
意味不明な means “other people cannot understand the meaning of what you are talking about, and they are appalled at you”. So, it’s pretty much the same as “nonsense”.