I followed the username validation in the src\XF\Pub\Controller\Misc.php file
For some reason the validation in the actionValidateUsername function returns empty, so the getPrintableErrorValue function (in src\XF\Validator\Username.php) returns the above result as if it were empty, Although he is a whole word.
I can't reproduce this. Can you provide your PHP version, whether or not the mbstring extension is installed (check admin.php?tools/phpinfo -- don't rely on the server environment report), your settings for the options below, and a Hebrew name which is not considered valid?