What was the חטא of the meraglim?

One approach is as follows. The leaders made the following calculation. Things were good in the midbar. They had a spiritual life with no effort. Manna came down from heaven every morning. If they had a question they asked Moshe who asked Hashem. In short, spirituality was handed to them on a silver platter. On the other hand they realized that in Israel life would change radically. They would not live a life of nissim, manna would not fall from heaven. They would have to work hard to retain their spiritually. If they had a question in Torah they would have to figure out the answer.

Based on the above, the meraglim decided life was better in the midbar and therefore did what they did.

What was their sin? They didn't want to work hard at their spirituality. They wanted everything handed to them on a silver platter.

The medrash quoted by Rashi in Parshas Shelach states on the pasuk ויבכו העם בלילה ההוא that it was the night of Tisha B'av and Hashem said that you cried for no reason, I will give you a reson to cry on the future. In other words, the חטא of the meraglim is the source of the Aveilus of Tisha B'av. Now we need to understand how does the חטא of the meraglim relate to the punishment? Where is the midda kneged midda?

Given the above we can understand the midda kneged midda. They wanted to have a life of miracles and easy spiritually. Hashem said, I am going to destroy the Beis Hamikdash on this day and go into hester panim. In other words, just the reverse of what the meraglim wanted. To be spiritual in Golus without the Beis Hamikdash takes even harder work. The Beis Hamikdash was a mini midbar. People could come and be spiritually recharged by the Nisim and the giluy hashechina in the Beis Hamikdash. Therefore Hashem says I am going to take even that away and send you into Golus so that you can fix the Aveira.