You may be seeing a suppressed exception at the end of the page. Could you enable the error handler and report the results? I can't reproduce this with the current CVS image.

Enable the error handler by adding "$g_display_errors = array(E_WARNING => 'halt', E_NOTICE => 'halt', E_USER_ERROR => 'halt', E_USER_WARNING => 'halt', E_USER_NOTICE => 'halt');" to your config_inc.php file.

I'm sorry but with the error handler enabled I have no more information. I have tracked the bug manually (by adding comment) and it seems that the error comes from the access_denied function.
This function is called by auth_get_current_user_id(): the MySQL query is "SELECT id FROM mantis_user_table WHERE cookie_string=''". This query returns no result so db_num_rows($result) is < 1 and the access_denied function is called.
Note: $g_anonymous_account = '';

I think that this has been fixed as a side effect of 0004995: Sign up for a new account no longer works...

By applying the patch described in 0004995, the problem is resolved but the administrator email is displayed (it is not a problem for me).