This issue occurs if Linux
proxy is selected in a backup job, when Veeam tries to hot-add a virtual disk to a VM which is deployed on the ESXi host with the CBRC option enabled.
Due to the VMware limitation, you cannot attach disks to VMs while they are powered-on.
Veeam Linux proxies have only one transport mode (hot-add) so far. That is not the case with regular Windows proxies.
According to VMware documentation CBRC feature was introduced in vSphere 5.1, thus may affect all systems with this version and above.