We have seen cases where someone or something has deleted the vpop3postgres account from Windows because they did not recognise it, and thought it may be a hacked account or something. This account is important to VPOP3 because it is the limited-rights account under which the VPOP3DB (PostgreSQL database) runs.
vpop3postgres should be an account on the computer where VPOP3 is running. If the computer is an Active Directory server, then the account should be in AD. If it's not an Active Directory server, then it should be a local user on that computer.
You will need to recreate the account either in Active Directory as a local user, as appropriate.
Create it as an account called: vpop3postgres
With password: Nc6ACboDt2jVL6
(It will work with a different password, but there may be issues on an upgrade)
You need to give that user full permission to the VPOP3\pgsql folder and subfolders
You can remove the user from the 'Domain Users' group (it is, by default, removed, so that it has no permissions to anything except the VPOP3\pgsql folder)
You will then need to edit the Windows 'VPOP3DB' service and reset the username/password to run the service as. This is needed even though the username/password hasn't changed, because the service applet will assign appropriate permissions to the user.