This is a known issue affecting Virtual Disk Development Kit (VDDK) 5.0 and 5.1. The issue is fixed in VDDK 5.1.2, 5.5 and in 5.0 Update 3.
After the lease is released, normal consolidation is possible. For more information on cleaning up the virtual machine state, see
Consolidating snapshots in vSphere 5.x (2003638).
To work around this issue, configure the backup software to attempt NBD directly instead of attempting SAN-based backup first, or ensure the correct block devices are present on the backup proxy server prior to attempting a SAN-based backup.
Note: Auto mode attempts SAN-based backup first then falls back to NBD if that fails. This situation will also trigger the issue.