For some unknown reason, I was also not able to convert my VMWare images using the VMWare vCenter Converter. I figured out that the problem was with a particular datastore on the ESXi server. When having a look at the datastore on the ESXi server, I discovered that the datastore was reporting ridiculously high capacities (eg. 600 TB; at one time it even went negative!!). I ended up enabling SSH on the ESXi server and tried to follow the instructions at:

http://www.blackmanticore.net/34ab986e70a5425091bcd879dfc98af4

These instructions didn’t work for me, but they did inform me of the fdisk command, of which I mucked around enough so that I was able to delete some partitions. I found that I had to manually delete partitions one at a time. I.e. press d, choose a partition, and then press w, and then would run fdisk again.

After this, I used Partition Wizard Home Edition to reformat all the partitions. I then re-installed ESXi, and everything started working again.