If the file manager works and this is the error you get when trying to use FTP, I think it just means you’re not using the right password.
Please remember that the FTP password is your hosting account password. This password is randomly generated for each account (by default), and is never the same as your client area password. You can view your account’s FTP password at any time in the FTP Details in the client area. If this password doesn’t work, please try resetting it through the Account Settings in the client area.
MonstaFTP doesn’t work well with large file transfers. It’s nice for a few uploads or downloads or quick edits, but for any bigger tasks a desktop FTP client works much better.
I have another problem. I try to install a forum phpBB and I have this message.
General Error:SQL ERROR [ mysqli ]User if0_35412184 already has more than ‘max_user_connections’ active connections  in file /home/vol3_5/infinityfree.com/if0_35412184/htdocs/3.3.11/phpbb/db/driver/driver.php on line 1031
It’s not WordPress, but a phpBB forum. The problem occurs during installation. So I tried to copy an already installed version, but it doesn’t work and I can’t see the source of the error. Is there a way to see the errors?