Did you receive an error when attempting a sync that read:
Error: Unexpected Exception = oauth_problem=token_rejected&oauth_problem_advice=The%20access%20token%20is%20not%20valid
This article will explain what that error is and how to resolve it.
What is it?
This error is thrown by Xero when the Token has expired. The Token will expire if it is not utilized in a specified time period. This means a sync must occur within the set time window or the token will expire.
Currently the time window that the Xero token will expire is at 7 days. It is recommended that you sync Order Time with Xero a minimum of once every 5 days to ensure that the token will not expire.