Hmm, that's strange. I've reviewed the code and it seems like the source VM does not have an instance uuid. I have no idea how this mat have happened.
You could confirm if that's the case - open the source VM's .vmx file and look for "vc,uuid".
If that's the case, you could eventually work around it by setting some uuid there (in a format like "50 17 3a 39 7d e6 c6 05-9c a9 1c bc c7 4d 47 f3"). However you'll need to restart the management agents on the ESX host! This is not a harmless operation, so do it only if you know and can accept its implications!
HTH
Plamen