@slaidman @KristineNeilStudio I had a similar issue in which I managed to find a work around. We were trying to migrating a Squarespace email list to a new Squarespace account however, I kept getting errors when re-uploading the csv to the new Squareapace site.
The fix that I found was opening the file up on my computer, adjusting the columns a bit (as they were a bit truncated on the sides), and re-saving the file (as a csv file). When I re-uploaded the file It worked. There were about 7000 emails in the list.
I hope this helps anyone else having a similar issue. I used a mac and also the "numbers" software to re export the csv file.
I definitely hope Squarespace can resolve this issue soon. Something is really off with the import list issues.
Cheers,
Mookey
www.mookeymedia.com