I was working today on a 2012R2 server that would give random error messages during the backup relating to registry permissions.
Unexpected error calling routine RegOpenKeyExW
In my case this was showing FRS and WINS VSS issues, but vssadmin list writers would show all error free.
In the end, to fix this issue, I checked further into the details tab of the error and could see that the Network Service did not have permissions to perform VSS operations. The fix, for me anyway, was as simple as allowing the NETWORK SERVICE account full permissions to the below registry key: