This error generally occurs for full syncs and could be due to the fact that accounting 3rd parties such as Xero limit the amount of time during which a sync can take place. For the case of Xero, this is approximately 30 minutes. If your MechanicDesk takes longer than this time to perform a sync of any kind, the access token used to authenticate with Xero will time out and Xero will no longer accept connections to their servers with that particular token.
The solution in this case is to lower the amount of data that MechanicDesk needs to sync. This can be done by setting the sync start date to a more recent date. Generally 3 months in the past is a good period of time. The longer the period of time into the past that MechanicDesk has to sync, the more likely the chance for this error to occur as the amount of sync data is increased.
We are currently working on improving our syncing algorithms to perform faster syncs with larger amounts of data.