549 - Sync token errors during a backup

Symptom

When performing a backup, the Agent displays an error "Invalid sync token found - ensure this is the only backup client backing up to this account".
A similar error will be seen on the Storage Platform as well: "Warn: GROUP\ACCOUNT - Backup: Sync tokens do not match. User possibly backing up from a different machine."

Cause

Every time a backup completes, a unique value is stored on both the Agent and the StorageServer. This value is sent from the Agent to the StorageServer on the next backup as part of the verification process. When this token does not match, it means that the last known backup does not match between the Agent and the StorageServer.

The following are typical scenarios when the error is seen:

Scenario 1
Cause
When reconnecting to an account, e.g. after a reinstall of the Agent. There will be no token present on the Agent.

Solution
Perform a backup. After a successful backup completion, the token will be updated.

Scenario 2
Cause
There was a problem on the Agent updating the cache (or the cache is corrupt/has been cleared).

Solution
Check the backup logs and resolve any issues that can cause problems during the cache update. Once resolved, perform a backup. After a successful backup, the token will be updated.

Scenario 3
Cause
More than one machine backing up to the same account. 

This can be spotted in the AccountServer or StorageServer logs. As seen in the example below, the backup comes from two different IPs and operating systems while the group and account names are the same. 

Example:
00:00:24.599 (25)  Info: GROUP\ACCOUNT- ASCommsManager: Backup client backup request from 172.23.0.33 (Type=SE, Version=7.0.3.583, Service Pack=0, Brand=A5BP, OS=Windows 2008)

02:56:40.097 (42)  Info: GROUP\ACCOUNT - ASCommsManager: Backup client backup request from 172.23.0.34 (Type=SE, Version=7.0.3.583, Service Pack=0, Brand=A5BP, OS=Windows 2003)

Note: The IP is not always a valid indicator, as the IP can change if an Agent is mobile or multi-homed. Also IPs may even be identical if the duplicate Agents both sit behind a NAT router. If a NAT router is present, it may be necessary to check the NAT session table to locate the original IP addresses.

Another indicator can be the Agent version or OS. However, note that the OS may appear identical. A common cause of this problem is a cloned Virtual Machine where the Agent has been installed previously.

Solution
Note: Data corruption can occur in this scenario, resulting in errors and large backups daily, as well as the inability to restore data.

Regardless of how the problem has occurred, the error will persist until the duplicate Agent is prevented from backing up.

Remove the Agent from the additional machine(s) or create new backup accounts and run a full integrity check on the affected account.