Ok - this one is odd. Have solved it, but thought it worth posting.
Create a personal dashboard portlet based on Project object
Solution: Reset multicast on affected server, as issue was not repeated when logging into different server.
However, still having issue with setting Filter Mode. Workaround is to select multiple OBS values as default, then change Filter Mode to "Unit and Descendants" As this is a personal dashboard/portlet, where I need multiple OBS values by default, this works fine.
Thanks for sharing this, Dale