no new connections after some runtime

Hi all,

I have a mini-pc running for some network tasks (FTP-Server, IRC-Server, mIRC-Bot, Windows-Folder-Share-FileServer, DynDNS-Updater, VNC-Server, ...). It is running since 2012, with a small software firewall to control network accesses.
Since I've set up this machine, I have an issue with it that occurs after it's running for some time:

The machine goes into a "bad network state".
In this state, windows itself doesn't signalize any error, I have some stable connections still running (e.g. users still connected to my IRC), BUT: I cannot open new connections.
This means: I also have my mIRC connected to some servers that it was still connected to when the error state has been entered. But I cannot connect to new IRC-Networks and user also can no longer connect to mine. I cannot establish new VNC-Connections. I cannot connect to FTPs from that machine or connect to the machine's FTP-Server form another machine.
In short: I can't establish new connections, but the open ones stay open.
Ping in and out works.

When I execute "netstat -a -n -o", I see a lot of connections with the status "ZULETZT_ACK". They don't disappear even when I kill the corresponding process.
Those "dead connections" are also visible in the overview of my firewall.

I can only exit that state by restarting that machine (so far).
Simply disabling and re-enabling the network interface of disabling the firewall doesn't solve the problem.

The error might be related to improperly closed sockets.
It appears much faster, when I excessively use its smb/windowsfoldershare server. (after ~1 week)
When I handled it carefully, it was able to run nearly a year without a reboot.


Do you have any ideas for a fix or work-around for this issue?

Best regards,
Rhino