I had this problem today as well. Clearing cookies didn’t help. Using a freshly installed browser (just for the truly paranoid) didn’t help either. (Sorry, Otto42.) What cleared things up was temporarily deleting my installed plugins files — and yes, these were turned off before upgrading. This allowed me to log in and upgrade. I then put the plugins files back, turned them on, and everything seems to be working fine.
Just for reference, the plugins I temporarily deleted were:
- WordPress.com Stats
- WordPress Database Backup
- Sphere Related Content
