been using Joomla and Mambo for a while now, deploying some free community sites and loaded most stable version of community builder. My only issue is that it does not let any of the users login.
I setup another new account and it worked. looking back I wanted to merge user data so I syncronized. Just for testing purposes, I cleared out all the accounts, recreated the issue, and sure enough after I synced I could no longer login using any username and password.
removed old accounts that were synced and readded and issue resolved. Seen several people having same issue after syncing now that I've had a few days to comb the forums. Great project!