TYPO3 Core

please read manual for proper configuration

Henrik is right. There is in fact a bug in the code. It's located in class.tx_felogin_pi1.php on lines 657-661:

// remove empty values
if (count($redirect_url)) {
return t3lib_div::trimExplode(',', implode(',', $redirect_url), TRUE);
} else {
return array();
}

However the problem is that a link to the front page IS EMPTY VALUE. Since obviously in a link to the front page there is nothing after the domain. So this piece of code ends up deleting the array item that contained the link to the front page (an empty value). I've successfully verified this by setting redirection to any other page, which does work as advertised. All is checked with a debugger so I am pretty confident this is in fact a real bug.