Why can't I trust a new node on a Spotfire Server? (The certificate could not be communicated to the Node.Could not trust nodes due to internal server error)

I've installed the node manager on the same server as Spotfire Server 7.7.

The node manager is running on different ports as required during installation.

I can see the node in Spotfire Server but get an error why I try to 'Trust' the node:   "The certificate could not be communicated to the Node.Could not trust nodes due to internal server error"

Does anyone know what this error means and how to fix it?

Many thanks!

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