[Radiant] Site 5 Install error

[keith]

All,
 
Finally got Radiant .6 to work on Site 5 yesterday - but when I now go to a
page to edit I get the following error:
 

Application error (Apache)

Change this error message for exceptions thrown outside of an action (like
in Dispatcher setups or broken Ruby code) in public/500.html
This happened yesterday too, I get it installed. It works fine, then the
next day I get the error. 
 
I need to install Radiant for around five sites in the near future and this
is driving me nuts (!). Can someone help me offline so I can overcome this,
I can't see why .5.2 was so straightforward yet this upgrade is causing me
nightmares!
 
Can any1 help?