ProxMox is not supported by Veeam at the moment.

The support is announced though:
https://www.veeam.com/blog/veeam-backup-for-proxmox.html

As Joe stated @MRieger , Proxmox was announced to be supported, but according to the official announcement on their blog site, it will not be GA until around Quarter 3 of this year (Fall sometime). You can read their official announcement below:

https://www.veeam.com/blog/veeam-backup-for-proxmox.html

Although, it doesn’t seem you’re working with Veeam & Proxmox directly, but using the Veeam Agent (so, an indirect approach)? Even so, there could be something about using a Proxmox-based VM which the Veeam agent is not able to read & thus the error. If no one else can post a solution, you may try and post something on the Veeam Forum.

I've seen this error a few times so far but no specific solution. On Reddit someone replied that changing from VM version 8 to 8.1 solved the issue, while someone else used pc-i440fx-8.0 instead. Maybe this is helpful to you, or if you find a different solution then please share it here.

Hi @MRieger - I just wanted to follow up to see if you were able to move the VM to Proxmox and get it working or not?  If you can post the answer here then mark best answer from above that helped you the most or your answer if none of them are relevant it would be helpful for other community members.  If there was no answer let us know and we can convert this to discussion instead of question.

Hi @MRieger  - I just wanted to follow up to see if you were able to resolve your issue with one of the posted comments or if you found another solution.  If you could update the thread and if one of the comments helped you mark that as best answer or if you found another solution post it and then mark it as best answer that would be great.  It will help others and the community to get answers/details for similar problems.

Hi,
I cannot answer for MRieger, but we had similar issues when we migrated 18 host with a lot of VMs from VMware to Proxmox.

Every time we had a VM which was original as BIOS - in Proxmox “SeaBIOS” - we need to change the “Machine” to 8.0. Else we would get the same “Failed to resolve ClientID. Machine doesn´t have identifiers” error and the VM would not boot.

Hardware → Machine

When using UEFI we can use Machine 8.1 and 8.0 with no issues.

Hope others can use this information, if they find this thread.

Hi,
I cannot answer for MRieger, but we had similar issues when we migrated 18 host with a lot of VMs from VMware to Proxmox.

Every time we had a VM which was original as BIOS - in Proxmox “SeaBIOS” - we need to change the “Machine” to 8.0. Else we would get the same “Failed to resolve ClientID. Machine doesn´t have identifiers” error and the VM would not boot.

Hardware → Machine

When using UEFI we can use Machine 8.1 and 8.0 with no issues.

Hope others can use this information, if they find this thread.

Thanks for sharing this and hopefully it helps the OP resolve the issue.

Hi @MRieger  - I just wanted to follow up on your question to see if you were able to get a reply that answered your question.  If you did please mark the one that helped the most as best answer.  If not and you solved it another way please let us know and you can mark your own answer for the thread.  Looking forward to ensuring you were able to solve your issue.