That error message looks incorrect, are you sure you have the right text?
Also, this points to a problem with the Sync Rule, so although you're doing the operation in the Sync Manager you would actually look at the FIM Portal to review the Sync Rule (assuming it is Codeless/Declarative and not Classic).
CraigMartin – Edgile, Inc. – http://identitytrench.com
Microsoft is conducting an online survey to understand your opinion of the Technet Web site. If you choose to participate, the online survey will be presented to you when you leave the Technet Web site.