I've been having this issue all day. Tried several solutions, won't work with even a single line invoice = same multi line error.

I RESOLVED my issue by:

1. Create new blank .csv file.

2. Type in new headers (I specifically did not copy them from the original s/sheet).

3.Copy all data from the original file to the new file.

4. Imported all records for me after doing this.

I suspect it is something to do with the excel spreadsheet (possibly the headers??) and not your data. Really hope this helps someone because I was very frustrated with it.