[@netweb Keymaster 12 years, 4 months ago]

If I login via http://localhost/wp-login.php I get logged in and returned to the home/front page http://localhost/

If I login via http://localhost/wp-admin the url becomes http://localhost/wp-login.php?redirect_to=http%3A%2F%2Flocalhost%2Fwp-admin%2F&reauth=1

If I login via a separate WordPress installed in /pref as a sub directory of the localhost install via http://localhost/pref/wp-admin the login URL becomes http://localhost/pref/wp-login.php?redirect_to=http%3A%2F%2Flocalhost%2Fpref%2Fwp-admin%2F&reauth=1

Note how the sub folder on my install is also /pref and is also part of the new redirect URL, for whatever reason, most likely a .htaccess issue or some other oddity in your config is is changing the redirect to /wp39.

With multiple WordPress installs this quite easily happens and you need to make sure you are loading the correct WordPress install you are actually intending to load 😉