http://tech.amikelive.com/node-41/comprehensive-solution-for-joomla-admin-session-problem-after-upgrading-to-joomla-1013/

and:  http://forum.joomla.org/index.php/topic,7452.msg1162667.html#msg1162667

One of the problems was the php versions. I had applied all the patches but to no avail. It kept giving me invalid session error which mentioned english.php (dont remember the exact error) . Finally I removed the closing php tag in english.php file(at the end of the file). That solved the invalid session problem.