I'm trying to import a collection of ~10000 cards using the CSV import. I've tried several times, but all I get back is "Error: The server encountered a problem trying to import your file. Please check that it is a plaintext csv file and try again.". I'm sure the file is a plaintext csv file, it uses the same format (headers) as the one I get from the export, and the same line endings (\n).
What else could be wrong? Could you fix the upload so it gives a slightly more detailed error message?

Make sure to quote anything that has a comma in the name or a colon in the edition, i.e.

1,,Ezuri, Renegade Leader,,,,,Scars of Mirrodin,,English
should be
1,,"Ezuri, Renegade Leader",,,,,Scars of Mirrodin,,English

and
2,,Fertile Ground,,,,,Duel Decks: Phyrexia vs. the Coalition,,English
should be
2,,Fertile Ground,,,,,"Duel Decks: Phyrexia vs. the Coalition",,English

if not, it will split these (or at least the comma one) into two, since the comma is the delimiter.

Thanks, that solved it. Also noticed you do get a detailed error message when the columns are correct, which is great!