Veeam for Microsoft 365 Invalid Serialized Stream, Unknown Opcode Found:4 error

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A recent Microsoft change to the Serialized Property of Exchanges items have caused “Invalid ‘serialized stream, unknown opcode found: 4’” errors across Veeam for Microsoft 365 users. This issue has been resolved with new hotfixes for users on versions and Below are the KB links to download the hotfix that pertains to your environment:

Hotfix for P20231218

Hotfix for P20231218

To install the hotfix, follow these quick steps:

  1. Ensure no backups, backup copies, or restores are currently running. If no jobs are running, stop the services for Veeam.Archiver.Service and Veeam.Archiver.Proxy.

  2. Extract the hotfix file, Veeam.Exchange.Serialization.dll, from the hotfix zip to a convenient location.

  3. You will be replacing the Veeam.Exchange.Serialization.dll in the following locations (It is highly recommended you either make a copy or add _old to the file name being replaced in the event you need to revert):

    C:\Program Files\Veeam\Backup365  

    C:\Program Files\Veeam\Backup365\Packages\Proxy

    C:\Program Files\Veeam\Backup365\Packages\Appliance

    C:\Program Files\Veeam\Backup and Replication\Explorers\Exchange

  4. Once the file has been replaced in all four locations, you may start the Veeam.Archiver.Service and Veeam.Archiver.Proxy services.

  5. Open up the Veeam for Microsoft 365 console, navigate to the Backup Infrastructure tab, then Backup Proxies, and select to Upgrade your proxies:

  6. Lastly, retry the backup job that has had been throwing the Invalid Serialized Stream, Unknown Opcode Found:4 error.

If you continue to experience the error, would like assistance with the hotfix, or have any other backup replated questions, please reach out to

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