Transferring multiple files ERROR

In my example, I am transferring somewhere beetwend 30 to 1500 files.
Sometimes it returns an error :
"Transfer channel can't be opened. Reason: An attempt was made to access a socket in a way forbidden by its access permissions.
Copying files from remote side failed. "

This kind of error does not appear often, 30% of the time appears.
We have tested the application (WinSCP) with manual transfer over 30 files and it return the same error.

How to fix this kind of problem ?

Two possible causes:
1) Your firewall/NAT is not configured correctly and does not allow the same FTP transfer connection port range as your FTP server is using. So once the server chooses to use a non-allowed random port, the transfer fails.
2) Your server runs out of free FTP transfers ports because it has a narrow range of ports allowed.

Or there yet another problem.

Please attach a full session log file showing the problem (using the latest version of WinSCP).

That is the reply from WinSCP.
The application is opening the connection and in a loop it retrieves files from that FTP.
Application closest the connection and after is opening again for other files from same FTP.
The application has 2 loops, before the second loop it opens the FTP connection and after the second loop it closest, this repeats until the main loop ends.

I'm sorry, but I do not understand the point of your response.

OK.
A manual test with WinSCP.

1. User opens WinSCP.
2. Connects to FTP.
3. Transfers 30 files to FTP.

Sometimes WinSCP is returning the same error, like in the first POST.

I understand the problem you have. And I've explained, what I believe is the cause of the problem already.