Steps to avoid these kinds of errors:
23 Aug 2017 15:36:24 ERROR Error creating a record for firstname.lastname@example.org. There's a problem with this state, even though it may appear correct. Please select a state from the list of valid states.: State/Province, code: FIELD_INTEGRITY_EXCEPTION
Error creating the record for email@example.com. You must enter a value, code: FIELD_CUSTOM_VALIDATION_EXCEPTION
Please check the "Retrieve state name instead of state code" option. This can be found in Step 2 of your Sync Profile.
If you enable "State and Country Picklists" in Salesforce it requires entered data to match the picklist values which is the state name.
For Constant Contact by default SyncApps retrieves state code. This option can be checked to retrieve state name instead.