People run into it all the time. You most likely forgot to copy the MachineKey section of your old web.config to your new web.config.

Hopefully you backed up the site then you can easily just copy over the old section, if not you'll have a few other options.

1. Rollback to a database backup/DNN version and do the upgrade again

2. If you only have admin users you can "register" with a new user, be sure to remember the password for that user. Then copy that new user's password/salt from the asp.net tables over your other users' password/salt to assign them that new password.

I'm glad that got you going Greg. Some people aren't fortunate enough to have a backup web.config file to roll back to!