How to fix "Invalid file. Please upload your file from Mint."

Before using Mint, I tracked my transactions manually in an Excel file. I came to Monarch from Mint, imported all of my transactions from Mint, and was very pleased. I would now like to extend my financial history in Monarch beyond when I started with Mint by importing my .csv from when I did things manually. I'm trying to use the Mint import but I keep getting the error in the title.

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I've tried to make sure all of my columns line up with the columns in the example file here (both in the example spreadsheet and in the "Importing Multiple Accounts" section), double-checked the dates were formatted the same, and I still get the issue.

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How can I import my old transactions from .csv?