ftp custom membership provider dll is never invoked

Hi!
I've implemented custom membership provider dll for ftp. It was working perfectly fine on windows server 2012R2 but it's never invoked on Windows Server 2008.

I've run out of ideas why. I'm afraid it have something to the with the version of the asp.net but I'm not sure. Any help will be appreciated!

 - I'm working on windows server 2008.
 - I've asp.net 4.5 installed on the server
 - I've installed this thing: http://www.iis.net/downloads/microsoft/ftp
 - Port 21 is open (anonymous logging is working just fine)
 - The dll: class library, target .net framework 4
 - the dll is added to GAC
 - I've registered new custom provider in IIS and connected it with ftp site. 
   It's registered as ManagedProvider. 
   FtpFormsAuthenticationProvider.FtpAuthDemo,FtpFormsAuthenticationProvider,version=1.0.0.0,Culture=neutral,PublicKeyToken=056911470e973009

    The token is correct -- I've checked it. 

The code:

    

    public class FtpAuthDemo : BaseProvider,
            IFtpAuthenticationProvider,
            IFtpRoleProvider
        {
            //Reading from config file did not work for some reason
            private string _path = "http://localhost:81/ftp/";
            bool IFtpAuthenticationProvider.AuthenticateUser(
                string sessionId,
                string siteName,
                string userName,
                string userPassword,
                out string canonicalUserName)
            {
                EventLog.WriteEntry("ftp", "DLL CALLED");
                canonicalUserName = userName;
                return true;
            }
            bool IFtpRoleProvider.IsUserInRole(
                string sessionId,
                string siteName,
                string userName,
                string userRole)
            {
                return true;
            }
        }

I'm using FileZilla as ftp client. The response is:

    Status: Connecting to ***...
    Status: Connection established, waiting for welcome message...
    Response: 220 Microsoft FTP Service
    Command: USER admin
    Response: 331 Password required for admin.
    Command: PASS ***
    Response: 530 User cannot log in.
    Error: Critical error
    Error: Could not connect to server