HMRC have also provided details of what some error messages relevant to MTD mean:-

• 403 (Client_Agent not authorised): the agent has missed a step in the agent journey or used the wrong credentials.
  
  This will usually mean that HMRC’s systems do not recognise that:
  • there is an authorised agent/client relationship
  • the agent has set up an agent services account
  • the business has fully signed up to MTD; or
  • incorrect credentials have been used when authorising the MTD software
• 403 (Forbidden): user is trying to submit a return for a period which is not yet closed.
• 404 (not found): user is trying to call back information for non MTD periods.
  
  This can be received when:
  • The request has been made before the business is fully migrated to MTD;
  • The request is for an obligation period that pre dates the business joining MTD;
  • Business is trying to submit a return to replace a paid Central Assessment on ETMP (the new VAT database). An error is returned because ETMP can’t process as no back end design in place; or
  • The request is asking to view liabilities or payments for a period that is not within MTD (e.g. the business may not yet have submitted a return so no liability has been generated).
• 500 (internal server error): user has not waited the 72 hours for their record to migrate to MTD (refers to the time that needs to elapse between signing up to MTD and filing the first return; HMRC advises also waiting until the email confirming successful sign up has been received).
• 8508 (InvalidTraderVRN): user has either tried to submit a return via the old XML route or it’s a genuinely invalid VRN