I was experiencing the same issue. The "Unable to obtain the lock on virtual disks" error occured after selecting the source machine and during the "retrieving source machine information" phase.
POCEH had the correct solution for me. My VM datastore is located in an NFS share with restricted access except for my ESXi hosts. After granting permission to the machine running the converter, I was able to complete the conversion setup and the conversion is currently running. Thanks for your help!