When generating a VAT control statement and selecting "Quick response to notice" (see screenshot below), Flexi requires the reference number of the notice.
The expected format is "99999999/99/9999-99999-999999", meaning the first number segment is eight digits long — if the first digit is a seven-digit number, it is necessary to add a leading zero at the beginning.
If you do not do so, Flexi will return an error message.
This number format is also required in the form for electronic submission of the VAT control statement.