This problem can have two symptoms:
This error happens if you try to access VPOP3 administration pages when
In the first case, simply log out, and log back in as an administrator
In the second case, this is because the access restrictions on Services → Webmail → WebAdmin Settings have been changed so that the IP address you are connecting from is not allowed access to the administration area.
In this case, you may have other IP addresses which are allowed, so if you can normally access the settings OK from another computer, you could log on using that computer.
Alternatively, the VPOP3 computer itself may still have permission, so go to Start → Programs → VPOP3 → Configure VPOP3 and see if that works.
For further options, see Reset Admin Access Restrictions