Maybe the password contains some characters which are handled differently by the backend (i had that problem with accented characters in passwords in joomla 1.0.12, and reported that issue to joomla core to be fixed for joomla 1.0.13, but am not sure if it was).
If that's the case, try a password with only A-Z, a-z, 0-9 and - and _ for now to see if it might be that.
Also clear your browser cookies.
Check also the "upgrade to Joomla 1.0.13" forum, there is a case of lock-out reported with Joomla 1.0.13 only (among other things). Read 2nd article on our homepage here for the link and more info.