Backups fails with the following error message:

The disk could not be opened. Error Code: 4 – A file was not found (Error code 'VMWAREBACKUPSTREAM_006')


The name of a  VMDK file attached to the virtual machine contains a non-supported unicode character.


Check the names of your VMDK files for the failing virtual machine and rename it in order to remove the non-supported unicode character from the name.

You can rename the VMDK without taking the VM offline if you make use of vMotion:

Search for: "Renaming virtual machine files using Storage vMotion"