i just fixed a problem, which seems to be plagueing for some time:
it is easy to reproduce:
i have tiki installed without having it in a subdir. like http://localhost/
i also have https auth enabled.
when i login another slash is added to the url, like
the problem lies in tiki-login.php:277
$prefix .= $http_domain . '/';
which is unnecessary in this case. i simply commented it out.
$prefix .= $http_domain; // . '/';
i hope this is ok and helps also other people.