Importing virtual disks of hosted formats in ESXi 5.1

Recently I stumbled over a new issue that really puzzled me. I was trying to import a virtual appliance (the Bitnami ownCloud stack) into my lab ESXi host. This appliance is provided with a virtual disk in 2gbsparse format to make it compatible with VMware Workstation. For using it in ESXi instructions were given to clone the disk into an ESXi supported disk format using vmkfstools like this:

   vmkfstools -i bitnami-owncloud-4.5.5-0-ubuntu-12.04.vmdk owncloud.vmdk

On my ESXi 5.1 box this command promptly failed with a very confusing error message:
  "Failed to open 'bitnami-owncloud-4.5.5-0-ubuntu-12.04.vmdk': The system cannot find the file specified (25)."

Of course the file existed in the current directory, so how could it not be found?!?

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