Error when backing up vmdk with GRT (V79-57344-38721)

Hi,

Im trying to backup several VM clients through the VirtualCenter and have the GRT Option enabled.

Except one client, all other VMs are backed up successfully.

The failing one shows the error above: "Failed to mount one or more virtual disk images".

If I back up the VM without GRT enabled, backup runs successfully.

My question:

Is it possible that my backupserver runs out of disk space when mounting the virtual disk for GRT? 

If the backupserver has to provide disk space for that process, can I calculate it?

Or is the required disk space the same size that the virtual disk has?

Environment: BackupExec 2012, all hotfixes installed

BackupDevice: Dedup-Folder