The import tells you what row failed to import if a specific row is what caused it to fail. It should say something like "Cannot import row ROW_ID because: ERROR_MESSAGE". Review what row failed then be sure that row has a username. If it has a username does it contain any possible invalid characters? Have retested import and am not having any issues importing new users.

Our import has nothing to do with CB Juice. I've no clue how CB Juice is designed. The way CBs works is it maps the data to the database objects and performs a standard backend user store for that object. It's more or less no different than saving them in CB > User Management directly. The first row of the CSV file is the field names and it's vital it exists and is correct. If it thinks no username exists it's possible you've tampered with the first row and have broke the mapping. The easiest way to import is to export a row then alter the users row while leaving the first row alone entirely.

I've no idea then. Could be an encoding problem. If using LibreOffice to open the CSV your settings should be as follows.

Character set: Unicode (UTF-8)
From row: 1
Separator Options: Separated by: Comma (uncheck all others)
Text delimiter: "
Other Options: Quoted field as text (uncheck all others)

You may want to try running User Synchronization within CB > Tools as your current user maybe missing in CB and is why it could be having a hard time constructing empty user object for import validation.