[FIXED] Veeam add Hyper-V host ‘Failed to connect to host’

When you want to add a non-domain joined Hyper-V host to Veeam, you get the following error:

Failed to connect to host 10.40.58.11

Server '10.40.58.11' does not exist or access denied.
Check firewall settings.

Possible reasons:
1. Invalid credentials.
2. Specified host is not a Hyper-V server.

Most likely is that you will have to add the hostname of the Hyper-V host to the username, like so:

HYPERVHOSTNAME\Administrator

Or else you can try:
KB1914: “Invalid Credentials” Error Adding a Hyper-V Host Using a Local Account