I can confirm that MikkoS ' solution works! (thank you!) open the .csv in Google Sheets, resave as a .csv and try uploading again. The error message should dissappear. Squarespace should work on this issue though; I was uploading a .csv file directly downloaded from Squarespace 7.0 site and re-uploading into 7.1 without any interference and it gave me the error message. Very confusing for users!