Did you make sure to retain your config.php file from your old moodle, and move it to your new moodle? I am not sure about the "session files". Even when I was running Moodle 1.9+, I don't recall needing to do this. Nope, I don't do anything like this in 2.4 either.
opps, I forgot to copy that over. I did now and it works. Sorry.
Make sure to keep of list of things to do when you upgrade so that you don't forget.
I think that you are getting the hang of this Moodle stuff, and maybe even starting to enjoy it a little.
By the way, just a hour ago, I upgraded my production Moodle 2.3 to 2.4. Everything went well.