Mailchimp - Your merge fields were invalid - Please enter a value

The PieSync Health dashboard shows the following error:
Altering default Mailchimp fields
You have the ability to rename default fields directly in MailChimp.
These default fields will no longer sync/update after altering the correlating tags in Mailchimp.
Solution
You can fix this by changing your fields back to the default setting:
- Go to your audience- Click on "View Contacts" > "Settings", and select "audience fields and *|MERGE|* tags" from the dropdown menu- In the column "Put this tag in your content:" change the tag back to the default values.
Change the first name field to FNAME and the last name field to LNAME (these are the default values)
Don't forget to delete your current connection and create it again.