I do understand what you are saying.
The problem is that the login form submits its results to a script within the wp-admin directory, which appears to be protected in some way. But there needs to be a way to submit the request to an unprotected form first, in order to access that resource, and that's just not there.
Namely, since that directory appears protected, and the only way to get to it is by logging in, it's a bit of a catch-22.
If you could log into another directory, then there would not be a problem - you would log in to (some other directory, which isn't protected), and then you would have access to the resource: In this case, wp-admin. But since you can't, it's stuck. You can't log in, and you can't access the way to log in. So one thing or the other will have to give.
Since we don't have access to the system, it's a bit of a guessing game for us here. There are a limited number of likely options as to why the directory may not be accessible:
- The directory may not have the correct permissions (generally 755).
- The directory may have authentication turned on (this is the file I mentioned in the previous post).
- Something else might be preventing access, such as a plugin (and it might use one or both of the other methods).
Since you have disabled the plugins, by renaming them, and you have checked the permissions, and you have checked for the existence of an authentication file, it's likely not any of those.
But by checking to see if a file is available to someone browsing (this is the CSS link I have posted), it means simply accessing the directory, or even a file in the directory, is causing problems.
A 404 error - not found - is being thrown. This is a server error. The "you must be" message is a part of that, and is likely configured somewhere on your server (possibly in your theme).
Ultimately, it means that the one issue you are having is that you can't access the wp-admin directory, which is what you need to do to login. Anything you submit via wp-login.php simply won't be processed until that is corrected, and that is what needs to be corrected.