If you see the following error message "Your merge fields were invalid.: ADDRESS - Please enter a complete address" you will need to check to see if your Address fields have been made required. If they are required, when you attempt to update the record in Chimpegration Classic, it does not allow you to save the record because the address field is incomplete.
In Mailchimp, when you import data from a CSV, the rules about required fields are not honoured. You can import without values that are required when entering data manually. However when you enter data manually or when data is added via the API (how Chimpegration Classic adds data), those rules are enforced. Even if you are not changing a first name or an address on the website or you have not mapped any of those fields in Chimpegration Classic, in order to save the record those fields cannot be blank.
In order to process data via Chimpegration Classic, you will either need to fill the required fields or you will need to change your settings in Mailchimp so that they are no longer required.
Other similar errors may include:
Your merge fields were invalid.: FNAME - Please enter a value,LNAME - Please enter a value