There's most likely a problem with your apache configuration.
If you have the interest and the time to learn how httpd configurations work now would be a good time to buy a good book .
If you don't have the time or the inclination, then the easiest way to resolve the problem might be to back up the username, then remove it from the system and rebuild it from DirectAdmin, and then restore your backups. If I were doing this I'd use the admin reseller backup option.
Note: I do not guarantee following my advise will keep your data safe. If you want mes to guarantee my suggestions, then pay us to carry out the work .