Due to the Xero API rate limits, you might encounter errors while attempting to send accounts from Core to Xero in a batch (Xero Sync Detail > Send menu). You can see the error on the Xero Sync Detail > History Sync Log.
oauth_problem = rate limit exceed & oauth_problem_advice = please wait before retrying the Xero api
This is not a sync error. You just need to sync the accounts multiple times to complete the 'Send' process because of the API limitations.
To learn more about the Xero API rate limits, check this out.