This error indicates that the FTP server was reached, but the login details were rejected. To troubleshoot, ensure the following:

1. The username and password provided are valid. Try copying and pasting the username and password into your FTP client to confirm.

This error indicates that the connection to the FTP server was successful, but the system could not navigate to the destination directory or did not have write permissions. To troubleshoot, ensure the following:

1. The FTP destination directory exists and is accessible using the FTP user.
2. The file path is relative to the login folder of the FTP account
3. The file path contains a trailing slash, eg: backups/
4. There is no preceding slash
5. The FTP user has write permissions to the destination directory

This indicates that WHMCS is unable to open a test file in the PHP system directory (as defined by the sys_temp_dir PHP setting) via the PHP fopen function. To troubleshoot, ensure the following:

1. A valid sys_temp_dir value is configured in your php.ini.
2. The PHP system temporary directory has writeable permissions
3. The fopen function is not disabled in the PHP configuration

This indicates that WHMCS is unable to write to a test file in the PHP system directory (as defined by the sys_temp_dir PHP setting) via the PHP fwrite function. To troubleshoot, ensure the following:

1. The PHP system temporary directory has writeable permissions
2. The fwrite function is not disabled in the PHP configuration