I’m getting the following error when uploading a Contact/Invoice etc:

This is a problem with the Authentication with the Xero API – you can just reauthenticate to fix this. When you authenticate with Xero some tokens are stored which are automatically included with each request to Xero – you usually encounter this error when these tokens have been cleared out, for example by opening another copy of fmAccounting Link (Xero Edition) and then closing it which will clear out the Xero tokens.