Have reviewed your install and on userlists and profile edit the username is set to NULL in the user object. This is being done incredibly early in the user object loading process, which suggests something 3rd party is acting on either Joomla user events or CB user triggers and setting it to NULL. The alternative is you've a database problem and that's causing this.
To check if it's a database problem first make sure your entire Joomla database has the same collation. You can't mix collations so it will cause query issues like this. If you've phpmyadmin the easiest way to fix this is navigate to the database and click "Operations" at the top then scroll "Collation" and select "utf8mb4_unicode_ci" (Joomlas default collation) from the dropdown then check the "Change all tables collations" and "Change all tables columns collations" checkboxes then click "Go". This will fix collation problems.
After being sure all collations are the same navigate to the _comprofiler table and be sure the following columns do not exist. They belong to _users and should not be in _comprofiler. Some 3rd party plugins or extensions for whatever reason insert these into _comprofiler when they shouldn't.
username
name
email
password
block
sendEmail
registerDate
lastvisitDate
activation
params
lastResetTime
resetCount
optKey
otep
requireReset
If you see ANY of the above in _comprofiler delete those columns.