SCVMM 2012: P2V/V2V - not enough diskspace due to disk reserves

Hi,

I'm trying to convert a Windows 2003 server from VMWARE to HyperV. In the Convert-wizard of SCVMM2012, the following error shows up:

As you can see, the check fails on 125972 - 123260 = 2712MB
If I uncheck one of the disks in the Convert-Wizard, the check succeeds

I have two CSV's with more than 1 TB space available.
My library-disk is 300 GB total, and 260 GB is still available. I have one library.

Where should I look to solve this issue?

Thanks in advance.