There are a couple of reasons that your login might fail for a particular server:
Check that you’re using the FTP login details provided by your server administrator or hosting provider. These will generally be different to the login details you use to gain access to website functionality or your website hosting account.
Caps lock is enabled
If caps lock is turned on, your login information might be in the wrong case. Check your keyboard’s Caps Lock indicator.
Excess white space
Even if you’ve copied and pasted your user ID and password, problems can still occur. A space before or after a user ID or password can creep in when you copy text from an email or file. These spaces generally won’t be automatically stripped by the server, because they could be a legitimate part of your credentials.
Firewall or antivirus is blocking FileZilla/FileZilla Pro
Some firewall and antivirus programs block file transfer connections by injecting false login failures. Check the settings on any firewall or antivirus scanning programs, or check with your network administrator to see if this might be happening to you.
Issue on the server end
If you’ve checked all of these possibilities and you’re still having trouble logging in, contact your server administrator or hosting provider.