I had gotten about 1500 cards of my collection entered. I was having difficulty remembering to put English as the language of choice. So, I decided I was going to remove all comments in regards to English.

I was doing it card by card (29 pages worth of cards).

This was tedious. I decided to export, then re-import.
Export was successful.
Import of the file was not successful.

At first, it was because there were extra columns in the export (cost, image/url, etc). I removed the extra columns.
Now I get the following error when I check file.
Error: The server encountered a problem trying to import your file. Please check that it is a plaintext csv file and try again.

Other than removing English from the Language column, and removing the extra columns, I changed nothing else. (Named file just Inventory).