Hi,
did you try to start the converter as a "Administrator"? Right Click and run as administrator?
But server 2012 is not official supported until now.
Check release notes.
https://www.vmware.com/support/converter/doc/conv_sa_501_rel_notes.html
Other oportunity if possible, add a second hard disk in the size of your belongings and copy the files to the new disk.
After that, you could delete the old disk.
Frank