MTD authorisation error

[abctech]

The HMRC Authorisation for MTD is only valid for about 15 months (and sometimes their website loses the authorisation earlier for no known reason).

If you suddenly receive Authentication error when trying to send VAT Return, the most likely reason is the authorisation has expired.
To fix this:
ABC VAT Return > OPTIONS tab > click CANCEL AUTHORISATION
and then do the Authorisation again (only takes a couple of minutes).