[Sam Bauers]

@hempsworth

You need to add the same SALT and KEY settings in bbPress config file…

define('BB_AUTH_KEY', '');

define('BB_AUTH_SALT', '');

define('BB_SECURE_AUTH_KEY', '');

define('BB_SECURE_AUTH_SALT', '');

define('BB_LOGGED_IN_KEY', '');

define('BB_LOGGED_IN_SALT', '');

.

Of course you need to actually set values in them to match your WPMU install.