I am not sure if anybody else had this issue recently. But I’ve seen some weird behaviour on one of our Veeam environments that is running Veeam 9.5 U4 and with VM’s that are using VMware Tools 10.3.10 or even 11.0.6. The problems seem to manifest on environments that are using ESXi 6.0.0 Build 6921384 in combination with the beforementioned VMware Tools versions and Windows Server 2019 Standaard as a GuestOS.

The problem arrises when you try to use the Veeam Application Aware Backup method on the Virtual Machine. The Application Aware part of the backup would fail with the message that there is an “Unknown Operating System” found, like below (I reproduced the message while trying the “Test Guest Credentials” option in a backup job -> Test Now:

Veeam Test Credentials "Unknown Operating System"
Veeam Test Credentials “Unknown Operating System”

Which is pretty weird right? Windows Server 2019 is supported in ESXi 6.0, but not an option to choose from the GuestOS list on a virtual machine. Starting from VMware Hardware version 17 (vSphere 7.0) you can pick this as an option. Since this is currently not possible we choose “windows9Server64Guest” which is Microsoft Windows Server 2016 or later (64-bit). This works fine anyway.

Having a look at the VMware side, I did notice something funny. While the VM is on the ESXi 6.0 host, the VM actually reports the following GuestOS:

Wrong GuestOS Identifier
Wrong GuestOS Identifier

Even looking this up on the PowerCLI side presented the above GuestOSFullName value:

PowerCLI GuestOSFullName information
PowerCLI GuestOSFullName information

This is actually not a supported, or even known GuestOS. So this might be the issue here. Next I tried to move this VM to another host. This host was version 6.7 build 16075168. And guess what, now the VM GuestFullName property was fine, it was now reporting: Microsoft Windows Server 2016 or later (64-bit) like it should.

Correct message in Veeam Test Credentials
Correct message in Veeam Test Credentials

As you can see, this now works fine. At the time I was troubleshooting this problem there were no newer VMware Tools versions available. So the resolution here is to move the VM to a ESXi 6.7 host. Once you did this, Veeam will be able to use the Application Aware backup again. If I had to guess, I would say one of the ESXi 6.0 patches is bugged if you are trying to use this in combination with a Windows Server 2019 GuestOS.

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