Backup Fails for Volumes larger 2GB

Hello,
We are using Server 2008R2 with Acronis 11.5.
The Server is equipped with 4x2TB Disks in Raid10. Unfortunately during installation (which had to go quick because of a server failure), we did not realize, that Windows will only use 2TB. But it is basically only a waste of space, our problem, not yours :-)

However, Acronis seems not to be able to handle this Situation correctly, regardless if we try with the OS built-in VSS or the Acronis VSS. The backup is peformed, but in 60% of the cases it says that the backup has failed. In the other 40% it says, that the backup was succesful, but shows a lot of warnings inside the email / logs.

attached is the log from the weekly backup performed last friday (in german).

We need some help getting this to work.
Thank you.