Thanks to this post: http://marcusoh.blogspot.com/2006/12/miis-attribute-not-defined-as-sourc…

I was getting a similar error:

icrosoft.MetadirectoryServices.AttributeNotDefinedAsSourceException: Attribute “mailNickName” is not declared as a dependency.
at Microsoft.MetadirectoryServices.Impl.EntryState.GetAttribute(String attributeName, IMacroCollectionBase collection)
at Microsoft.MetadirectoryServices.Impl.CSEntryImpl.get_Item(String attributeName)
at Microsoft.Exchange.DirSync.RulesExtensions.RulesCommon.IsAttributePresentInCSEntry(CSEntry csentry, String attrName)
at Microsoft.Exchange.DirSync.RulesExtensions.RulesCommon.GetOnPremiseObjectType(CSEntry csentry)
at Microsoft.Exchange.DirSync.RulesExtensions.MAExtensionObject.Microsoft.MetadirectoryServices.IMASynchronization.MapAttributesForImport(String FlowRuleName, CSEntry csentry, MVEntry mventry)

However, in my case, even though the mailNickname had a flow rule, it still errored out. By removing the attribute from the MA completely, I got the error to go away.