[TYPO3-english] help! locked out of back end

[David Cake]

	I have a typo3 installation.
	it was recently upgraded from 4.0 to 4.2.1, started life as a 
3.8 install.
	I cannot access the back end at all.

	Any attempt to access sitename.com/typo3 or 
sitename.com/typo3/install gets a very brief database error, then a 
redirect to
http://sitename.com/typo3/typo3/install/index.php?mode=123&step=1&password=joh316
	which, of course, shows the
>The Install Tool is locked.
>
>Fix: Create a file typo3conf/ENABLE_INSTALL_TOOL
	screen.
	The install tool is not locked, and ENABLE_INSTALL_TOOL 
exists (though message doesn't change either way). There is 
definitely a database specified in localconf.php, and it looks 
perfectly valid to me.  I have a working install on the same machine 
using the same source (and same mysql install).
	I have seen one or two differences between the two installs, 
largely due to history, but I have fixed all I can find, and still 
one works and one does not.
	I've combed localconf.php for differences between the two, 
and found nothing that looks likely except different sets of 
extensions.

	I've got no idea even how to debug this situation. I'd really 
appreciate some help.
	(hosted on Debian stable, using the debian stable installed package)
	Regards
		David