FTP user can not connect to the proxied server on ispconfig multiserver with a reverse proxy

I'm using an ispconfig multiserver with a proxy server at front and some servers at the back in a private network
When I create an ftp user on the proxy server(master server), the user can connect to it, but when I change the server to a backend server(proxied server) or create a new ftp user on it, the server responds "530 Login authentication failed"

This is the output of the FileZilla logs:

Status: Connecting to MyIP...
Status: Connection established, waiting for welcome message...
Status: Initializing TLS...
Status: TLS connection established.
Command: USER user1
Response: 331 User user1 OK. Password required
Command: PASS ********
Response: 530 Login authentication failed
Error: Critical error: Could not connect to server​

Can anybody help me?

 

My best guess is your credential as that error is common for it.

 

My credential doesn't have any problem because as I said, when I change the server to front server without any changes in the username or password, the user can connect to it without any problems but when I change the server to backend server, "Login authentication failed" error occurs

 

I would check whether I am connected to the backend server and whether I did the proxy right for ftp.

 

Thank's for your reply ahrasis
I solved my problem. I forgot to set the NAT rule for FTP connections.