I found that by editing sapphire/security/Security.php

line 179

from;
if($member = Member::currentUser()) {
$member->logOut();
}

to;
if($member = Member::currentUser()) {
//$member->logOut();
}

fixed the problem in blackcandy theme & showed useful output to the user.

I'm having the same problem, however I've gone into the Security.php file and made the suggested changes, however now it tells me "I'm sorry, but you can't post to this forum until you've logged in. If you want to log in as someone else, do so below. If you're logged in and you still can't post, you don't have the correct permissions to post." while telling me who I am logged in as. I am in the administrator's group, I've tried changing the permissions in the CMS so that anyone can post, but it won't allow it. Any other suggestions? Thanks.

---I am using External Authentication and wonder if that is what is causing the problem? Also, I don't get a "registration" option. When I go to the forum page, it knows me even though I am not in the Forum Members in the CMS (I added me as a forum member in the CMS but it made no difference, still no access).

I also have done the suggested fix and have the same result as the last post -- how to fix this???