Following POCEH's suggestion, you can convert one machine in several conversions as a workaround.
- Select only a few volumes for the first conversion, including the boot and system ones.
- After this conversion finishes, select the other volumes and convert to another VM. Deselect the 'reconfigure' check box in advanced options.
- After the second conversion finished too, use vSphere client to attach its resulting virtual disks (vmdk files) to the first VM. Remove the second VM but take care not to delete the vmdk files.
This is not a recommended procedure and should be done with care, but if nothing else helps...