You're likely trying to synchronize too many users at once. The error is saying you've used all of your maximum of 64M. Please from herein ensure you're using CB Login module and CB Registration so your users do NOT go out of sync. You may need to temporarily increase your memory limit to as much as 132M or more to ensure all your users can be synchronized. It's also possible you've a plugin consuming too much memory and can try disabling plugins 1 by 1 until resolved.