It appears that you have to install a viewer from realvnc ($? or registration?) in order to connect.
So I logged in to the raspberry via ssh and started a virtual vnc screen:
vncserver-virtual -Encryption PreferOn -Authentication VncAuth
This starts a new, virtual vnc server with the standard vnc authentications, tightvncviewer can connect to this one.
From this vnc connection I started a realvncviewer session to "localhost"
Accessing the options on the :0 realvnc server via realvnc viewer from a virtual (:1) realvncviewer from tightvncviewer.
From the options change Security/Authentication from "UNIX password" to "VNC password".
Now it is possible to connect directly to the realvncserver from tightvncviewer.