I was busy in finding fix of a bug in Joomla! 1.0.15 last weekend, there are some workarounds I found in official forum but still took me a lot of time to try each of them. It is a PHP default session’s save path problem:
http://forum.joomla.org/viewtopic.php?f=198&t=271541
http://forum.joomla.org/viewtopic.php?f=198&t=283649
Comprehensive Solution For Joomla Admin Session Problem After Upgrading To Joomla 1.0.13