Login probelm after upgrade

Try the following:
1) Make sure you clear your Joomla cache after the upgrade. Our Javascript files have changed, and those need to be refreshed.
2) Try clearing your browser cache (shouldn't be necessary, but never hurts).
If neither of those work,
3) Disable the "Show login popup" option in the Login/Logout area of the JFBConnect configuration. The Squeezebox error you mention would be from this option, though it shouldn't prevent the login from happening. We're investigating why the popup isn't working on all sites. It seems to be on those sites that are forcibly removing Mootools (a Javascript library included with Joomla). If you know that your site/template is no allowing Mootools to load, that would be part of the problem.

Let us know which step (if any) helps!
Alex

Phil,
Thanks for the update. We're looking into an issue with that popup that is happening on just a few sites. Not sure what the cause is, but we're hoping to have it fixed for the 4.2.1. release later this week. Obviously, leave it disabled until then.

Thanks for the report,
Alex