VMWare Plugin Backup Error: Failed to open virtual disk (4043271)

Backup of a Virtual Machine fails with the following error message:

Error 2011/06/01 09:49:52 530 Data Plugin NVClient Failed to open virtual disk

Module/Thread Id Code Time Message
VDDK :06264 141 0 84938 Remote connect done
VDDK :06264 36 0 84952 Failed to open vmdk
VDDK :06264 138 0 84952 VDDK error encountered: code 13
VDDK :06264 139 0 84952 Retrieving error message text
VDDK :06264 52 0 84952 VDDK error message is 'You do not have access rights to this file'

Error 2011/06/01 09:49:52 530 Data Plugin NVClient Failed to backup data of VM disk '[Esx_Data_00] vm-name/vm-name.vmdk' to media
Error 2011/06/01 09:49:55 530 Data Plugin NVClient Failed to backup VM 'vm-name' uuid '00000000-0000-0000-0000-000000000000'