Hi
I can guess of two possible reasons:
1. You use Converter 5.5.0 or older. If so, please use the latest Converter 5.5.3. There was an issue related to disks larger than 2TB resolved in 5.5.1
2. Some of the source volumes (in the LVM group) is nearly full. Extend it a little on the destination. 10% for a smaller volume or less for the large 3TB one.
If none of these works, please upload the log bundle for further examination.
HTH
Plamen