Can't access admin.pl after upgrading to 1.2.0RC5

["Robert Fraser" <..hidden..>]

Hi

I have upgraded to 1.2.0RC5 and I now get "Error! access denied" when I try to access admin.pl

I can log in and everything else looks OK (apart from some minor errors which I seem to have sorted), but I will eventually need to get into 
admin.pl :-)

Can anyone give me a hint how to fix or where I should look to see what is wrong?

Thanks
Rob