Mailchimp Upload email addresses from Google Sheets Problem

I am looking for someone who can troubleshoot syntax errors while uploading records from a Google spreadsheet to Mailchimp.

I have uploaded from my spreadsheet (which has about 600 email addresses plus other demographic data) to this list many times, and suddenly I am getting a message indicating "8 syntax errors" and I can't identify where the problems are. The Mailchimp help pages don't provide a lot of troubleshooting detail, and since I use the free version of Mailchimp, there is no customer support. :-(

In addition to fixing the problem, I would like to understand how to troubleshoot it myself should it ever happen again.