Error replicating when virtual machines or datastore names containing special characters
VirtualMachine is null or empty is a generic error for virtual operations that are not able to find a virtual machine, there are a lot of possible causes for that -most of the times the VM is simply deleted-, but this issue could also appear if the virtual machine or concerned datastore names contain extended characters.
This issue is seen when special characters like: (, ), %, &, *, $, #, @, !, \, /, :, *, ?, , <, >, |, ;, ' etc.
VMware indicates that usage of special characters in VMware objects is also not advisable.
A more detailed information in this article of VMware Kbase.