I have no idea. I can't see any reason for that in the code. I've retested usernames with spaces, UTF8 characters, email addresses, and single alphanumeric words. They're all working fine across 2 different installs (1 development and 1 clean production). Only thing I can think of is something is acting on the CB login triggers or Joomla login events that's targeting that user for whatever reason. You're not able to reproduce this with any other user?