V9.0.1 Anonymous Author in Conversations

I have just upgraded from 8.5.7 to 9.0.1…
I am using the Conversations feature but since the upgrade the ‘Author’ of each message is now always shown as ‘Anonymous’, both in the front and back ends. It is saved correctly in the database but Concrete is not displaying it correctly. (It works fine in 8.5.7)

So I then tried a clean test install of 9.0.1 and it is has the same issue.

I assume this is a bug, or am I missing a new setting somewhere?

I have just done another clean install of C5 on another web server. Same bug occurs. The Author’s name just appears as “Anonymous”…