I have seen this issue before and know exactly what it is. It is related to your servers
SESSION_SAVE_PATH which is not working correctly ... It is not STORING the session.
How do I know this? See what happens when I try to login with INCORRECT username/password:
However, see what happens when I try to login with the CORRECT username and password admin/admin. The login actually WORKS, but then when it redirects to the X3 panel, your server has already forgotten the SESSION, and therefore the page redirects back to the login page again, but without notifications about wrong login of course.
By FTP, please check if you can find a file /panel/filemanager_errorlog.txt. OR if possible, you can check your server error log? Ultimately, this is an issue you will have to report to your hosting and they will have to fix. It's sad that some hosts can't setup their servers correctly. If you have FTP for me, I would create a small test script that simply shows that sessions are not stored. You cannot login to X3 panel if your server does not save login-sessions.