This error arises from an issue with HMRC's MTD system and is entirely beyond the control of VT Software.
This error may indicate that HMRC's MTD system is busy. Please try again later or early the next day. Only a specific minority of businesses appear to suffer, but for them the error can occur repeatedly on a particular day.
If there is any possibility for confusion, please check the correct business is open in VT and check the VAT number.
If the error persists, please contact the HMRC online services helpdesk.
This error indicates HMRC's MTD service is not available due to maintenance.