If you're certain your login credentials are correct your server may not allow outbound FTP connections. If your client and FTP server are the same, you can use localhost as your host address.
If you see the error An unknown error occurred during file save when saving files in the editor, your host may be blocking POST content that it deems suspicious, for example, when it contains the names of PHP functions. Please see this guide on how to address this issue.
If your user interface is showing language variable names instead of their values, e.g. DOWNLOAD_ACTION instead of "Download" and you're running Monsta FTP on IIS you may have a MIME Type issue. IIS must be configured to serve .json files with application/json MIME Type. Microsoft has a step-by-step guide on adding MIME types available here.
PHP's magic quotes feature was deprecated as of PHP 5.3.0 and removed as of PHP 5.4.0. Having magic quotes enabled may affect Monsta FTP's ability to operate normally. There are advanced methods for disabling, but the easiest way is to add a directive to an .htaccess file with:
php_flag magic_quotes_gpc Off
If your date/time stamp in your Monsta FTP file listing is out of sync with your local time, this will be because of the timezone setting on the remote FTP server. Unfortunately Monsta FTP is unable to manipulate the date stamp from the FTP server as it does not return a corresponding timezone when returning file listings.
Restricting folder access is not something you need to do via Monsta FTP. When you (or your webhost) set up FTP users you should normally have the option to set a home directory. This can be different for each user, so they have their own file space. Monsta FTP will log users into whatever their home directory is set to, and will restrict them to this specific folder.
If running Pure-FTPd and you experience issues with Monsta FTP not running, you should run the following in your console, which will ensure the correct language character set is used in the client:
This is a common problem on Windows as increasing the upload limit requires a different process. Please see this tutorial.