The cause of this is a contact in your Google Contacts that has only a year for a birthdate, so is missing the day and month.
You can open the Google Contacts web interface and search through them. If you find the one, add a day and month. Then restart eM Client and it will sync correctly. But there may be more depending on where they were originally created. Google Contacts don’t allow just a year.
We have provided a workaround for this which will be in the next 9.2 release.