[TYPO3] CHC Forum and 500 server errors

[Another Vision]

Hi List
 
I have a Typo3 4 installation that's running sweetly with one odd glitch.
 
The site has CHC forum running (currently with almost no traffic). I can
view the forum, log-in as a frontend user, post messages etc etc. All good.
However, if I login as a backend user, then try to view the frontend forum
pages in a separate browser window I get:
 
500 Internal Server Error
 
The site's running the recommended 16M php memory limit, and tests and
operates fine (without the 500 error) on my test server locally also at 16M.

 
Any ideas what might be causing this on the live server? 
Thanks
Mike