Ran into this issue for a customer running Veeam BnR 9.5 Update 4b, some quick google-fu found the following Veeam forum thread; https://forums.veeam.com/vmware-vsphere-f24/vddk-async-operation-errror-2-t58591.html
As described by Andreas Neufert from Veeam, Virtual Disk Development Kit (VDDK) is provided by VMware which is leveraged by Veeam to perform backups and other functions. In this instance, there appears to be a bug introduced in VDDK after VMware introduced “a faster way to process data over Network NBD mode (async processing)” which is causing certain VMs to fail during backup jobs.
The recommendation from Andreas is to ensure the underlying ESXi hosts and vCenter are updated to the latest builds from VMware or Veeam support can add a registry key on the VBR server which disables the new VMware processing.
Veeam will always recommend calling support to obtain the reg key, this is to ensure you are applying the registry tweak for the right reason. However, if you are confident that you are affected by this issue, the details for the reg tweak have been provided below.Continue reading