FWIW - I'm experiencing the exact thing. I want to move all the VMs from an older ESXi 4.1 host to a newer 5.1 host. All but 2 have gone just fine. But the final 2 won't go for this same reason. When you try to select them as a source, you receive the "A general system error occurred: Invalid argument" error in the Converter wizard.
I suspect the reason the logs don't help is because there has been nothing to log - selecting the source VM is the first step, and that won't complete.
There appears to be something about these 2 VMs that causes an error immediately upon selection. I haven't been able to figure out what so far.