The Torah states (Bamidbar 13:31):
But the men who went up with him said, We are not able to go up against the people; for they are stronger than we.
On this, our sages comment (Sotah 35a):
Rabbi Ḥanina bar Pappa says: The spies said a serious statement at that moment. When they said: “They are stronger,” do not read the phrase as: Stronger than us [mimmennu], but rather read it as: Stronger than Him [mimmennu], meaning that even the Homeowner, God, is unable to remove His belongings from there, as it were. The spies were speaking heresy and claiming that the Canaanites were stronger than God Himself.
Rav Moshe Feinstein (Darash Moshe al HaTorah, ArtScroll, p. 243) (1) explains that saying that the sin of the meraglim was lashon hara, is puzzling, because:
In fact, they committed a much more serious transgression of disbelief in Hashem by stating, as the Rabbis teach us (Sotah 35a), that the people of Canaan were stronger than Him, G-d forbid. It would seem that the sin of corrupt character traits manifested by speaking evil without any gain - as stated in Koheles (10:11): וְאֵ֣ין יִתְר֔וֹן לְבַ֖עַל הַלָּשֽׁוֹן, and the slanderer has no gain - leads the more grave sin of denial of the power of Hashem.
The Daf Yomi Digest writes (2):
The reason you feel cold about Eretz Yisrael is not that it is the land’s true identity, since nothing could be further than the truth. You feel cold because you are spiritually apathetic! If a Jew who goes to Eretz Yisrael can’t find any good attribute which he can report, this bears a very telling witness about the true state of his spirituality!”
In Tehillim 78:22, it says:
For they did not believe in G-d, and they did not trust in His help.
So, it seems that the sin of the meraglim was not merely lashon hara, but the disbelieve in G-d. Not trusting Hashem, despite the great miracles that G-d did for us, and even saying that "they", e.g. the Canaanites, were "stronger than Him".
In Kovetz Ma'amarim, Rav Elchonon Wasserman cites the verse "and that you seek not after your own heart and your own eyes, after which you go astray" (Bamidbar 15:39) and explains that it is "mans nature" to have faith in G-d, but that it can be affected by our external influences. That's why the Torah warns us explicitly not to "follow our hearts", e.g. our temptations.
According to the Ramban, the sin was that the "spies" added their own conclusion and added to their own "job description". Moshe Rabbeinu said that they needed to observe what the land looks like "and what the land is that they dwell in, whether it is good or bad; and what cities they dwell in, whether in tents, or in strongholds" (v. 20).
I highly recommend reading these articles, here and here.
(1) Klugman, E. M., Rohr, P. O., & Rosenberg, A. Y. (1994). Darash Moshe I: A selection of Rabbi Moshe Feinstein’s choice comments on the Torah (1st ed.). Mesorah Publications Ltd.
(2) Chicago Center for Torah and Chesed; November 30th, 2015; Daf Yomi Digest for Maseches Sotah; Daf 35