I’ve set up WPMU with subdirectories, rather than subdomains. I was having issues logging into multiple blogs e.g. http://www.blog.com/blog2

I found that removing the following from the wp-config.php file

define('SITECOOKIEPATH', '/wp-admin');

define('COOKIEPATH', '/');

sorted the issue and hasn’t broken sitewide login and cookie support.

Thought it may help someone

[@detective Member 15 years, 5 months ago]

Here’s another tip:

I mentioned in another thread that I had problems logging in WordPress. If someone logged inside WordPress, the cookies were valid in both WP and BB but the user was unable to post in BB. The problem was that BB was authenticating the username in the cookie against a lowercase version of the user_login value. I manually changed all user logins in the database to their lowercased version and everything worked correctly