How do we explain the mistake of the Meraglim? How could such chashuv people, "kulam anashim," have been so mistaken in their calculations and speak badly about Eretz Yisrael? R' Tzadok HaKohen says an amazing pshat in Likueti Ma'amarim (p.85 -- link). The Meraglim were telling the truth! The Meraglim had great neshomos and were therefore able to sense the future. They saw that life in Eretz Yisrael would eventually lead to a churban, would end in a tragic and painful to galus. Midrashim already in Parshas Braishis predict 4 galiyos; Midrashim tell us the galiyos were revealed to Avraham; 9 Av was revealed already to Ya'akov. The "bechiya l'doros" of the Mergalim was a cry in advance of what they knew was inevitably coming.
So what did they get wrong?
Because "b'shuv Hashem es shivas Tziyon hayinu k'cholmim" means that all that sorrow and pain is just a passing dream and not reality. It may look real, it may even feel real as it occurs, but it is not eternal truth. (R' Tzadok here is echoing a torah of the Ishbitzer in Sefer Braishin in which he distinguishes between emes and emes l'amito. The Meraglim spoke truth, but it was not the emes l'amito, eternal truth of Torah.)