I was under the same impression and did already read the provided link beforehand.
The only thing I was confused about, was the mention of "unknown cluster", because there is no Cluster.
It is a bit strange, but I don't have the necessary access to further investiage until the customer gives me the appropriate data and will open a case if neccassary.
I have this in sametime environment, where you use loadbalancer in front or you trying to use sametime sso with a different hostname of the domino server.
Also you can check if the user has a sametimeservername inside his persondocument; this is using to redirect the user to this sametime server after login (sametime homeserver).