Thanks everyone, I still have no idea though how to go about fixing this problem

It does appear to only be happneing to a few of us who are added to the most number of roles (we've probably got about 6 or 7 different ones to limit people to different forums)...however if I remove a few roles from my user who gets the error it still makes no different, still get the critical object error on all the discussion forums.

The only way I've been able to get around it on my other troublesome user was to completely delete this user, have them register then add them to all the same security roles again for the forums and now they logon and access them just fine...its so strange, and not really something I want to have to do every time this crops up.

Does ANYONE have any further ideas???

Ta in advance!