Backup with Veeam failing (Disk Size=null)

[caroline]

Hi everyone,
we're evaluating PVE as replacement for or ESX environment.
Currently, we're struggling with Backups via Veeam failing because of volume size being reported as "null".
So, the Veeam support asked me to run the API call they use for getting the machine information via my local browser.

REST Response from Browser:
{"data":[{"ctime":"1767620148","format":"qcow2","content":"images","vmid":1000,"size":540672,"volid":"BLN:vm-1000-disk-0.qcow2"},{"content":"images","format":"qcow2","ctime":"1767620149","volid":"BLN:vm-1000-disk-1.qcow2","size":53687091200,"vmid":1000},{"content":"images","format":"qcow2","ctime":"1767620150","volid":"BLN:vm-1000-disk-2.qcow2","vmid":1000,"size":4194304}]}

REST response received by Veeam:
2026-02-13 09:51:35.3466 00010 [22308] DEBUG | [ProxmoxNode][pollux][34323550-3939-5a43-4a44-303330303846]: ==> Response "Get" "https://xxx.yyy.zz:8006/api2/json/nodes/xxx/storage/BLN/content?vmid=1000", "code: 200 - OK", duration: "178 msec", headers: "[]", cookies: "[]", body: "{"data":[{"volid":"BLN:vm-1000-disk-0.qcow2","vmid":1000,"size":null,"content":"images","ctime":"1767620148","format":"qcow2"},{"ctime":"1767620149","format":"qcow2","content":"images","vmid":1000,"size":null,"volid":"BLN:vm-1000-disk-1.qcow2"},{"content":"images","format":"qcow2","ctime":"1767620150","volid":"BLN:vm-1000-disk-2.qcow2","vmid":1000,"size":null}]}"

Can anyone assist me on where to start troubleshooting here?

Thank you very much in advance,
Caroline