I am not sure if I replied to this other thread or missed it.
If it is not working for underscore, it is most probably caused by some validation code. Ajax registration does not do anything to stop a user from using _. Is there any chance that you are using a plugin or code that puts restrictions on username/email or validates them and avoids underscore.
The ajax registration plugin is incapable of doing it as we simply use validate_username and is_email which are both WordPress provided validators.
Please check and let me know.
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