Admins Can't See Thier Dashboards After Upgrade - Any Ideas?

I just got a call from an admin of a few sites on our WPMU network because they log in, go to their list of sites and when they click 'Dashboard' for any of their sites, they end up back at the login screen.

I've read a few things about this on the web but thought I'd ask among fellow MultiSite users before I dive in...

Any ideas?

Cheers.

Martin