The most common cause is that you have reinstalled the software or changed to a new PC.

The envelope number can be any 6 digit number, but one that has not been used before as you as a filing agent. If you open the accounting period for the client you are submitting and go to Tools / User setup / Transmission and change the envelope number, try 100000 as an example. The XML gateway ID does not need to be changed.

Click ok and then regenerate your accounts, i.e. click final accounts and then either reduced accounts or micro reduced accounts and then try the submission again. The system will then auto add one to the envelope number for each submission.

If you are running the software across a network with multiple users, and all PCs have been changed, you will need to amend the number on any machine that will file to Companies House ensuring there is a gap between them, e.g. 100000, 200000, 300000 etc.