[TYPO3-english] can't access back end - going crazy....

[David Cake]

	I have one install (of at least two on the same machine using 
the same source) where I can no longer access the backend. I think it 
started out as a 3.8 install, and was just recently moved to 4.2.1. 
The front end actually continues to work fine.
	When I attempt to access the back end via 
http://sitename.com/typo3, first it briefly shows a database error, 
and then redirects to 
http://sitename.com/typo3/typo3/install/index.php?mode=123&step=1&password=joh316
	and displays the 'The Install Tool is locked.' page.
	Given that the two are using the same source install, I can't 
see anything that suspicious on typ3conf/localconf.php, any ideas 
what might be going on?
	This is driving me crazy. I've ben tring to work out the 
problem for a couple of days now with no luck.
	Regards
		David