The WordPress user name field does not have default validation other than checking if it exists already. What security plugin are you using? What validation are you performing with the form? Do you have a default value set?

Default settings are being used throughout. Regards security, I don't have any set up as its a testing site

I've deleted the original form and created a new one, and appears to be working so, now resolved.

I have the same. Everything pretty vanilla on a dev site. Out of the box, "username invalid" for no apparent reason.

I had the same problem with some registration forms.
I fixed it with this:
 
function wscu_sanitize_user ($username, $raw_username, $strict) {
    $username = wp_strip_all_tags ($raw_username);
    return $username;
}
add_filter ('sanitize_user', 'wscu_sanitize_user', 10, 3);
 
The function makes wordpress less strict in its requirement for usernames.

I havpre-existing accounts whose usernames violate Formidable's requirement of only lowercase and no special characters. I need Formidable Forms to grace these existing accounts since they cannot be modified. Either that or just allow users to create usernames with special characters.

This is critical since my existing users are using a new member registration form to update their profile after they have logged in with a username which violates the Formidable requirements.

Thanks!
Joe