The skins already check for this, so it only applies to API created Users. Usernames were allowed to have upper case characters. This caused issues with the mail system, as it automatically converts usernames to lower case. The converted values didn't match the true values, thus confusing login errors appeared that the account didn't exist. The fix is simple, in that it simply checks for upper case characters and blocks it. It does not re-define what a valid User is though, as it's possible existing Users exist with upper case characters, so we're not going to block those. The block only happens as User creation time, so only Users creation attempts will be affected. Existing Users will be fine. Note, if you're transferring a User to a new box, you'll need to rename the Username.tar.gz to username.tar.gz (or user.admin.Username.tar.gz to user.admin.username.tar.gz), as Users are created during a restore, which applies with the block.