I've been playing around with a test installation, and noticed if you set up another blog for a user and he/she was to abandon their first blog by assigning it to another user, every time you logged in or clicked admin, it attempted to send the user to their original primary blog resulting in error.
I can't figure your trac system out, ie how to register, so thought I'd post it here.
Forgive me if this has been reported and/or fixed in a nightly, I couldn't find it through search if it has.
Anyway if your having this problem, I couldn't see any visible way from the site admin end to change the DB, please correct me if there is a way.
ok I'm using PHPMyAdmin here, and I'll assume you have some knowledge of DB management, it's pretty easy anyway.
You will need to know the UID of your user, then go to the DB find the table: wp_usermeta in there you need to find the ones relating to the UID you want to change, in my case it was UID 2, there is then one containing the metakey: 'primary_blog' and meta_value will contain the primary blog id. Simply change that to the users new primary blog id and your done.
Please be sure of what your doing before changing any values, perhaps back it up first to be sure as I take no responsibility for any mistakes you make.
Anyway this has worked for me. :-)