Can't connect to host periodically

[bhart]

We are using Filezilla to upload files to a machine tool control. The PC and the control are on the same subnet internal to our LAN.

The user connects FTP. Here is a successful connection log.

Status: Connecting to 172.16.2.50:21...
Status: Connection established, waiting for welcome message...
Status: Insecure server, it does not support FTP over TLS.
Status: Server does not support non-ASCII characters.
Status: Logged in
Status: Retrieving directory listing...
Status: Directory listing of "/NCDATA" successful
Status: Connection closed by server

Several times a week when the user tries to make this connection, Filezilla fails to connect. The message log simply shows that Filezilla could not connect to host.

However, if I go to a different PC and try to connect using Filezilla, the connection works.

To clear the problem, it is necessary for the user to reboot the PC. This always fixes the problem. Simply restarting Filezilla does not fix the problem.

We are using Filezilla v3.42.1 The PC is running Windows 10 Pro 64 bit.

Any suggestions?

[boco]

What is/are the used security software program(s)? How many connections pro IP are allowed by the server?

My suspicion is that you might accumulate stale connections to the server in the systems, until, at one point, either the server is not accepting any new connections from that IP, or, the limit of half-open connections is reached and the pipes are clogged.
Alternatively, the firewall or similar software is giving in at one point.