Just found out:
My problem was even simpler:
I instaled MD5 Insecurity and Mouldy Old Cookies on folder bb-plugins when I should had install it on folder my-plugins.
I change them to the new folder, delete again the line
$bb->cookiepath = ‘./wordpress’;
on bb-config.php
And it is still working.
I was having the exact same problem, but with normal WP (not WPMU).
My problem was that I instaled wp and bbPress both in subfolders, like:
http://www.mysite.com/wordpress
http://www.mysite.com/bbpress
The problem was on cookies, since bbPress was creating them on root and wordpress both on root and on ./wordpress – but it seems that the ‘good cookie’ was the one on ./wordpress.
The ideal solution would be to set WP to use cookies only on root, but I wasn’t able to do it in a simple way. So I configured bbPress to set cookies on ./wordpress.
This was done by simply adding a line to bb-config.php:
$bb->cookiepath = ‘./wordpress’;
Now it seems that all is working fine.
Hope it helps you too