A little background:
I've written a scheduled task on my staging server that grabs user data from AD and either creates or updates users in Kentico based on that information. I then sync the user data from staging to production, which has been working fine until this morning. The AD sync to staging works fine, but when I try to sync staging to production, three new user accounts fail with the following error:

	Synchronization server error: Exception occurred: [DataConnection.HandleError]: Query: UPDATE CMS_User SET [UserName] = @UserName, [FirstName] = @FirstName, [MiddleName] = @MiddleName, [LastName] = @LastName, [FullName] = @FullName, [Email] = @Email, [UserPassword] = @UserPassword, [PreferredCultureCode] = @PreferredCultureCode, [PreferredUICultureCode] = @PreferredUICultureCode, [UserEnabled] = @UserEnabled, [UserIsEditor] = @UserIsEditor, [UserIsGlobalAdministrator] = @UserIsGlobalAdministrator, [UserIsExternal] = @UserIsExternal, [UserPasswordFormat] = @UserPasswordFormat, [UserCreated] = @UserCreated, [LastLogon] = @LastLogon, [UserStartingAliasPath] = @UserStartingAliasPath, [UserGUID] = @UserGUID, [UserLastModified] = @UserLastModified, [UserLastLogonInfo] = @UserLastLogonInfo, [UserIsHidden] = @UserIsHidden, [UserVisibility] = @UserVisibility, [UserIsDomain] = @UserIsDomain, [UserHasAllowedCultures] = @UserHasAllowedCultures, [UserSiteManagerDisabled] = @UserSiteManagerDisabled WHERE [UserID] = @UserID: caused exception: The UPDATE statement conflicted with the REFERENCE constraint "FK_CMS_UserSettings_UserSettingsUserGUID_CMS_User". The conflict occurred in database "KenticoIntranet", table "dbo.CMS_UserSettings", column 'UserSettingsUserGUID'. The statement has been terminated.

If I create a new user through the Kentico GUI it syncs fine.

Any thoughts on what might be going on here?