PCI scans will fail if IPs are removed from your account.
Previously scheduled scans do not change IP scopes when IPs are removed.
When you remove an IP from a PCI account you will receive a warning informing you that removing an IP that is referenced in a previously created scheduled scan could cause the scan to fail.
This behavior is expected. When you create a scheduled scan for PCI and tell it to scan ALL hosts we put a copy of all of your current host IPs into the schedule. When the IPs are removed from the account it does not update this schedule. This causes the service to try and scan an IP that is no longer registered and causes the scan to fail.
To resolve this, delete your existing scheduled scan and create a new one. You can either specify the hosts or set it to ALL again as it will now only contain the IP's that are once again registered to your account.
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