Currently i have exactly the same problem...
All internal paths ( like /home/blablabla... ) was changed,
while the URL is the same,
and i simply copied few days old backup,
with changing configuration;
first i did the update 6.02 to 6.03 but now it's the same
with all old files too ( 6.02 )
The path in main.inc.php is correct,
i already deleted all cookies related to the forum,
even deleted the U_SessionId of the admin user
at the same time after closing browser,
but i still get the same
"You must login first.
To access this section you must first be logged in. You will be taken to the login screen shortly."
It also is not connected especially to Admin link,
whereever i click after login
( i'm on "Home" page after login ) -
i got the "Login" not "Logout" link on the top,
what means the forum counts me as not logged in

The issue seems to be quite old,
is there any solution already ?