Hey Juan,
Ah! The following probably happened:
1. Sync subscribed a WP user to your list
2. User unsubscribed from MailChimp list
3. Sync runs again, sees that user previously subscribed (but doesn’t know user unsubscribed later on) and tries to update that user. Sync will then run into an error.
There are two possible actions you can take. The first would be the same solution as was mentioned in this topic on the same subject. You take this route if you don’t want them to be subscribed again.
If you do want them to be subscribed again, I kindly ask for your patience. We’re currently working on a fix for this. If you’re interested, you can follow our progress in this GitHub page.
Hope that helps! If you have any other questions, please let us know!