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Have you clear your cookies on your browser since upgrading? in xoops 2.4 the cookies dope or domain is set correctly with a 'multisite' approach so if you goto xoops at
http://www.yoursite.com and the same copy of xoops at for example secure.yoursite.com you will still be logged in.
In previous copies of xoops there was an old tracker bug that cause xoops to have a log in error, i think you all remember it in 2.3 those that where using it and earlier.
this has been fixed by the addition of some code that allows for the base of the domain to be determined where if it was set as this without the period before it, it will not allow you to be logged in anywhere within the subdomain of the same site the moment you are there.
This is in the beginning of common.php but anyway at the moment you probably have a cookie for both installation versions both 2.3 and 2.4:
// Cookie Domain/Dope conversioning differences!
yoursite.com // 2.3 and earlier (Auto Assigned and Buggy not Set)
.yoursite.com // 2.4 and later
clear your cache and tell me how you fare?