PROBLEM

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')

CAUSE

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

SOLUTION

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: https://kb.vmware.com/s/article/1029513

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