In the Confluence Datacenter installation documentation a note needs to be added that the default user (that's used when using the Confluence Windows installer and choosing the option "Install Confluence as a service") needs to be changed to a different, dedicated user when setting up the cluster mode, because the default user is a local user and cannot be used for folder sharing on a different server.

Customer was aware of Atlassian's documentation about changing the user account - Creating a Dedicated User Account on the Operating System to Run Confluence
However for all of their previous (un-clustered) Confluence Server installations, this was never needed, they all ran fine using the default user.

It looks like this is not possible for clustered Datacenter installations, so it would be very helpful to point that out under "requirements" in Installing Confluence Data Center
and in the "Set up and configure your cluster" section of Set up a Confluence Data Center cluster.