What sin did Ezra mourn over in Ezra 9?

Answer: Marrying people of the land

Leaders told Ezra that many Israelites, even priests and Levites, had married women from the surrounding peoples.

“After these things had been accomplished, the leaders approached me and said, “The people of Israel, including the priests and Levites, have not kept themselves separate from the surrounding peoples whose abominations are like those of the Canaanites, Hittites, Perizzites, Jebusites, Ammonites, Moabites, Egyptians, and Amorites.”— Ezra 9:1 (BSB)